I don't want to talk about the legends of Gatecrash yet, though that's coming, I swear.
Return to Ravnica's mechanics ranged from not worth it (Scavenge, Unleash) to occasionally relevant (Detain, Populate) to amazing (Overload!) and Gatecrash promises to offer a similar range of usefulness.
The mechanics are here, and the FAQ is coming.
I'll list these from least-used to most-used, at least in my opinion/guess. As of now, only 67 cards are spoiled and I reserve the right to say, "But this card is amazing and should go in any deck of these colors!"
Bloodrush - Gruul (Red/Green)
(Pay a cost, discard this card: target attacking creature gets a bonus until end of turn.)
I wanted to like whatever mechanic showed up for the Gruul Clans, and I do. I'm a big fan of these colors and of decks that feature a lot of creatures. I can't wait to build some deck with 37 creatures and 23 lands in Standard.
That being said, I don't think this is going to do enough in EDH games to make a difference. I've seen decks with high numbers of pump spells, and they are usually some verion of infect or trample-get there for 21 general damage deck.
If Bloodrush was a spell, and not an ability, then it's possible that some deck with Wild Ricochet and Fork would take advantage, but that's not going to work either. Alas. I like the flexibility, but very few EDH games will hinge on Bloodrush--unless we get one that gives Double Strike and Trample, a la Runes of the Deus.
Evolve - Simic (Green/Blue)
(Whenever a creature enters the battlefield under you control, if it has higher power or toughness than this creature, put a +1/+1 counter on this creature.)
I want, so very very badly, for this to be overpowered and amazing. I've got two decks, Animar and Kraj, that are going to snap up a lot of Evolve creatures...and then slowly take them out again, I suspect. The problem is that these creatures are competing with, no kidding, some of the best creatures ever printed for space in your deck, and most of them are not going to live up to that billing.
For an evolve creature to be good, it has to come down early, and then you need to play other, bigger creatures for it to grow. There's nothing wrong with a creature-heavy deck, but for these to be good, they REQUIRE lots of other creatures to come into play. Realistically, you're better off just playing a better creature to start with.
Here's the caveat: Simic gets a lot of tricky creatures, and Fathom Mage is an effect I love to have in a creature-heavy deck. I already play Garruk's Packleader and Primordial Sage in lots of decks, so having a third--that gets bigger!--is always going to be fun. We'll have to see what other awesome Evolve cards show up.
Battalion - Boros (Red/White)
(Whenever this creature and at least two other creatures attack, get a bonus effect.)
This ability has the potential to be impressive in EDH. A lot is going to hinge on the quality of the base creature and the bonus granted. Some of the spoiled cards are absolutely worth it (An angel that throws a Lightning Helix! Clerics who give indestructibility!) and I'll be talking about those in my set review column.
I'm not sure how effective it will be, though, considering that I rarely get to attack with a group of three or more creatures more than once or twice, unless some haste or indestructible tricks are happening. There's just too many board-wiping cards to count on Battlion being active.
Cipher - Dimir (Black/Blue)
(Cast the spell, then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)
Rules Question: Can you encode onto something that isn't always a creature, like a Keyrune or a Creeping Tar Pit? Yes, you can. As long as it's a creature when your spell resolves, then it doesn't matter if that stops being a creature, you can make it a creature again and get the Cipher effect. Go to town, Tar Pit!
Free spells are inherently dangerous. We've got some very strong evidence of that in Magic's past. Palinchron is one of my 'I wish they would ban you' cards, because it goes from free to infinite so easily.
Cipher is as close as they want to get, I imagine. It requires a creature to be in play, and that creature to deal combat damage. I'm surprised that they would print Cipher cards while Invisible Stalker is in Standard, but that's the main interaction I'd be aware of.
In EDH, it's a lot easier to be unblockable in some way, especially in Blue/Black. I fully expect to see lots of Cipher cards in the right color of deck, especially once the more powerful cards are spoiled. It's going to be a fairly common thing, and I've already seen enough decks with a theme of 'unblockable' to know that the Cipher cards will be added.
The lesson, as always: Have answers for creatures.
Extort - Orzhov (White/Black)
(Whenever you cast a spell, you may pay . If you do, each opponent loses 1 life and you gain that much life.)
First of all, the rules question. Can you play Extort cards in a deck that isn't B/W? The answer is yes. Believe it or not, mana symbols in the reminder text doesn't count towards determining color identity. See Rules Manager Matt Tabak's Twitter feed on December 31. So you can play Extort cards in a wider range of decks, and brother, you're going to.
Sucking one life from each person at the table and gaining three, for the low cost of , is a real bargain. So far, they have only given us creatures with Extort, but heaven help us if this ability is on an artifact, enchantment, or (gulp) a land. You'd see that land in almost every deck that could play it, and rightfully so. If the ability is just on creatures, then it won't be too bad, because creatures die a lot. Exort stacks, though, so if someone gets two cards with Extort out, they can pay the twice and gain 6 life.
The good news is, someone playing Extort cards is very quickly going to earn the ire of the rest of the table, so they will get attacked for their impertinence. Good times! Even with that, I do feel Extort is going to be the mechanic that sees the most play in EDH games.
One more thing that applies to all of the Return to Ravnica and Gatecrash mechanics: Dragon's Maze, the third set, is probably going to add mechanics to new colors, or do new things with old mechanics. There's a good chance that we'll see a lot of really amazing cards.
Stay tuned, as it's time to talk about the Commanders of Gatecrash!