I'm been loving Blue/Black recently, so I've been looking forward to Dimir in Gatecrash.
Then I saw the Cipher mechanic reveal, I think I almost fainted from delight. That looks really awesome and fun to play. There's now no way whatsoever that I'll miss the pre-release.
I've been thinking about Orzohv too. I'm not incredibly excited about the mechanic, but some of the cards look pretty sweet (Deathpact Angel, for one) so I might play orzhov at a second pre-release.
What sorts of things are you looking forward to in Gatecrash?