Seems like a simple question, right?
There's a lot of factors that go into this evaluation, questions we have to ask about the card in order to determine if it goes into 'most' EDH decks.
Before we go into those questions, I'd like to take a moment and salute those of you who deliberately build around terrible cards. Overpriced Equipment, rampantly underpowered creatures, or sorceries that might as well say "Target player asks, 'Why bother?'" If you have found a home for a bad card, that's a story and a triumph all in one.
Eric Levine, in his recent article, talked about submissions revolving around Zedruu the Greathearted. He used my wife Elizabeth's Bronze Bombshell trick, which is fun, but not nearly as amusing as when she gives away Darksteel Relic. We had a 30-minute discussion on which sexually transmitted disease the card should be altered to portray, since once you get it, it's very difficult to get rid of. A guaranteed source of life and cards for Zedruu is not to be trifled with.
So, what makes a card worthy of an EDH deck? I think that if it can yes to at least one, and preferably more, of these questions, you can at least consider adding it to your 99. These questions are roughly in order of importance, but it doesn't have to pass the first two in order to still be useful.
Question #1: Is it fun? Does it fit your theme? Is it shiny?!
Perhaps the most relevant question of all is three little words: Is it fun? I fully realize that fun is not measurable, or defined the same by all players. This format is the one where we are encouraged to do things that are enjoyable to us, even if it's not the most effective use of a card. To those who pick a theme, or a tribe, or an artist, or whatever build-around you're going with, this is the primary consideration. If the card fits, it might stay. Sometimes there's more choices than there are slots (Zombie decks!) and sometimes there aren't quite enough (Noggles!) but you will choose what cards you can to make the best deck possible while retaining the theme. We also get soft spots for foil/foreign/signed/altered versions of cards that might be strictly inferior to other cards, but it's OUR special card and it stays. I'm not immune to this, and you probably aren't either.
Example: Telepathy. The card does nothing but expose the nefarious plans other players are holding. I view this card as a lot of fun, even though it doesn't do anything about the cards. It's also amusing logistically, because if someone has 30 cards in hand you're going to need a bigger table.
Question #2: Is it ridiculous in combination with my general?
The choice of your Commander is literally defining the deck. It will be the creature who gives a bonus, an effect, an ability, or just represents what the deck is about. Once you have that choice, then it's time to see what cards put a smile on the face and a song in the heart. Some decks use the general sparingly, as a figurehead. Some decks save the general until later, or some play it turn 2 and attack repeatedly. Whatever your strategy is, you'll need some cards that combine very well--maybe even infinitely well--with the Commander and then you can really go to town.
Whichever strategy you choose, there's probably cards that make your general amazing. Some of those will be universally useful yet still sneaky-good, like Lightning Greaves in a Bruna, Light of Alabaster deck. If your general is there for late-game tricks, make sure you're playing cards that get you to that late game. Turn 6 Damnation followed by turn 7 Thraximundar can be pretty backbreaking.
Example: Corpsejack Menace in a Skullbriar, the Walking Grave deck. You can talk about Curiosity and Niv-Mizzet, the Firemind, but that's obvious. Adding some +1/+1 counter tricks to a Skullbriar deck is super-tasty frosting on an aggressive, face-beating deck.
Question #3: Is it an effect I want? Is there synergy with the rest of the deck, not just the general?
We have no shortage of cool things to do in EDH, even after allowing for different definitions of 'cool'. But we want cards that play well together, to have a whole that is greater than the sum of its parts. Sometimes this leads to insane six-card combos, sometimes it is a small yet consistently powerful effect. The dazzling plays in EDH sometimes come about because there are synergies we don't expect. There are cards that literally beg to be played with other specific cards (Konda's Hatamoto, or the Sword of Kaldra, Shield of Kaldra, and Helm of Kaldra) but it comes down to your deck and your playstyle.
A word of caution about synergies: Don't be that player who takes 20 minutes a turn because you have a Reveillark loop going. Especially don't be that person who doesn't see the way to turn that into death for the other players. There is little that irritates a table more than sitting around, waiting, waiting, waiting, and finally we are dead. Guess whose head is now wearing The Target Hat in the following game?
Example: Quicksilver Amulet and Sneak Attack in a Kaalia of the Vast deck. Kaalia provides a way to cheat on mana costs, and your deck will therefore play huge, overcosted creatures. Kaalia rightfully draws a lot of hate, so you need backup ways to drop your 8-cost Angels, Dragons, and Demons into play.
Question #4: Is it a threat that is difficult to answer?
This is separate from your general being hard to kill, such as Avacyn, Angel of Hope or Lazav, Dimir Mastermind. (Though, in the right colors, those are awesome cards to have in the 99!)
Threats can be any number of things, from big, hexproof creatures, or something sneaky, like Chalice of Life flipped and making a player lose five life a turn. Perhaps you're playing an Enchantment deck, and your Honden of Infinite Rage is pinging away twice a turn because of Paradox Haze. Or your threat can be a Blightsteel Colossus, promising to send you packing if it gets in once.
Whatever threats you choose to run, creatures without trample, flying, or other evasion aren't high on EDH players' boards, unless the deck is built to take advantage of such with Brion Stoutarm or other such effects. I once had a Multani, Maro-Sorcerer deck that contained every way I could find of giving a shroud creature trample, because Multani was big and had shroud, but he'd get chump-blocked on almost every attack.
Sometimes, your threats are things that make all of your creatures terrifying, like Collective Blessing. Perhaps your threat is a Planeswalker building up to an ultimate. If you have an offensive card that isn't easy to get rid of, then you have a card worthy of being in your EDH deck.
Example: Assemble the Legion. Sure, it's an enchantment and can be dealt with that way, but it dodges mass removal of creatures and has no mana cost to keep generating more and more hasty soldiers. If you haven't had the fun of ticking this card up, I strongly urge you to do so.
Question #5: Does it answer another player's threats?
I have lost the following EDH game: He plays his general (Ashling the Pilgrim) on turn 2, turn 3 it gets one counter, turn four Umezawa's Jitte, equip, attack. I could never keep a creature in play. I had answers, but I never drew them. This told me that perhaps I needed more answers in the deck.
There is a fine line regarding how many answers you need to be running, versus having a deck with a plan of its own. I once had a deck based around Mangara of Corondor, dedicated to stopping other people from doing what they wanted. It was a frightfully boring deck to play, because 90% of the cards were reactive, needing other players to do something before it would have a target.
We toe this line by having flexibility and repeatability, points I'm about to cover. It also helps to have tutors that can find answers. Sometimes, a card can be both an answer and a threat at the same time, such as Phyrexian Rebirth. We clear the token player's board AND get a huge creature. My answer is now a threat--sweet!
Example: Condemn. Or use Swords to Plowshares. For one white mana, it's great to have an answer to most problematic creatures. I'm a bigger fan of these two than Path to Exile, because ramping them a mana is a real bonus for that player.
Question #6: Can I use in this multiple ways?
This question spawns a lot of "It's so good!" responses. We have maximum flexibility on one card: Vindicate, but even 'Destroy target permanent' as the text isn't good enough sometimes. There are very few cards which only have one use, and since we have the entire history of Magic to choose from, we get the best spells, the best creatures, and the best creatures that act like spells. We have split cards, we have modal spells like Cryptic Command, we have kicker spells like Illuminate. Giving multiple options on a card is how we make sure that every card is useful in the deck. We don't need to use restrictive cards, unless that's a personal preference/deck theme.
Tutors fall under this category, because as long as we planned ahead, searching for the right card is easy for the game state. Sunforger in a Zedruu deck can fetch, at instant speed, Counterflux, Congregate, Boros Charm, or any number of useful spells. With or without tutoring, the ability to have one card give us threats and answers is remarkably poweful.
Example: Merciless Eviction. This is insanely good for EDH. The different modes ensure that you will always have something relevant to do. If there's a game where you need to exile different types all at once, then it sounds like your game got appropriately out of hand. I love that I can really mess with the five-color, thirty-planeswalker deck.
Question #7: Is it repeatable?
If answering yes to these questions is good, being able to say yes turn after turn is truly remarkable. That's part of what makes the Titan cycle so amazing: you get the effect when it comes into play, and then again when it attacks! Visara the Dreadful or Avatar of Woe are similar, and have the additional effect of "If you love the creature in your hand, better not play it yet!" which can lead to so many fun games.
The repeatable effect is half of why Equipment is so popular in EDH: It can go on any creature you control. (The other half is that most of it is colorless and therefore can go into any deck) Rancor is good because you can use it over and over, making any creature into a trampling, significant threat.
Repeatable effects are rarely spells, though. Sprout Swarm and other Buyback spells like Capsize are the exception, not the rule. Mostly, you'll find these effects on creatures, artifacts, and enchantments--permanents that can be dealt with more easily than a Buyback spell.
Example: Elder of Laurels. An all-star in my token deck, he messes up combat math something fierce. With eight mana, the Elder + six other creatures, one big attack and no interference, your opponent is going to take at least 14 damage unless every creature is blocked. Up the creature count to ten (flashbacked Increasing Devotion?) and it's 22 guaranteed damage, at least!
Question #8: Is the cost appropriate?
This is not only asking if a card has a high or low mana cost, but to the other effects associated with a spell. Some spells like a Fling require a sacrifice of a creature, or Scorched Ruins can put you up by two mana as long as you're willing to lose two lands, etc. This cost can also be an untap cost, a cumulative upkeep cost, an activation cost.
Mostly, we don't mind huge investments of mana to cast our spells. Tooth and Nail often ends games, as does Omniscience. We are all aboard the big-spender plan. Other costs have to be weighed in that context, especially when something costs us more mana to use after we cast it. EDH decks want to use all of their mana, and don't often want to tie up our mana.
Example: Lord of Tresserhorn. Four mana gets you TEN POWER, and regenerating power at that! The life loss is no big deal, the opponent drawing two mana is a bit painful, but his two-creature snack means that you'll have to stock your deck with cheap creatures if you want him out on turn four. His cost is significant, but somewhat manageable.
Question #9: Does it scale up to multiple players/targets?
We tend to play EDH in multiplayer mode. The French 1v1 is not a format I enjoy, because your deck requires such specific tailoring. Wizards did not care much about multiplayer until fairly recently, Syphon Soul notwithstanding. But in these last few years, we've seen cards and mechanics that were designed for or tweaked for multiplayer experiences. The new Primordial cycle is an excellent example, as is Extort. These cards are explicitly better when there are more opponents!
Would you play Terastodon over Sylvan Primordial? Would you play both? Neither? They offer different types of scaling. The new Overload mechanic scales very well and is pretty brutal in EDH, especially Cyclonic Rift. I am not looking forward to seeing Vandalblast played when Mycosynth Lattice is out.
Example: Comet Storm. Perhaps the ultimate in scaling, this card requires that you know how to count. It's less math than Fireball, and more of a powerhouse than Rolling Thunder. If you have lots of targets, and the mana to do it, then it's pretty straightforward. I like that for the same 22 mana, it'll hit one player for 20, or two for 19, three for 18, and so on.
Question #10: Will it eliminate a player quickly if not dealt with?
Really, the key is that word 'quickly'. I don't want to hear about your Invisible Stalker deck with no enchantments or equipment. As players, we don't get frustrated when we die to 40 1/1's attacking us in one turn, but if you manage to kill us with the same 1/1 attacking 40 times, now we are ready to flip a table and set a deck on fire.
Some decks are all about big creature after big creature, with nothing to buff or boost them, and that's fine--there's an elegance to saying "Can you deal with my 9/9 turn after turn?" There's room in most EDH decks for big creatures, but they have to be BIG to be good enough. Is your six-mana Craw Wurm any good in comparison to Silvos, Rogue Elemental? This is often where the word 'durdle' comes up, because something huge like a Krosan Cloudscraper is just going to walk over and stop at the first little creature unless he has help. It's just durdling around, hoping for a chance to get in for damage.
Example: Lord of Extinction. I did a spit-take when I saw how expensive this card has become, and it's all due to EDH players. For five mana, you get something that's not only enormous late-game, it can keep getting bigger! It has no way on its own to get past Saprolings, though, so that's why he's huge but needing help.
This time, I think I covered all the bases. Ten questions to decide if a card is good for Commander games. It's not too hard to say "That would be good in EDH" but the real trick is "What card(s) are worse than this card in my deck as it is?"
Editing and updating your Commander deck is a much more difficult prospect. I'll take that on soon, though. See you soon!