Gatecrash previews have officially started. All the guild mechanics are out in the open. There's a bunch of cards already spoiled, and the set looks superb so far. I'm looking forward to the prerelease.
Top guild right now: Orzhov. Extort looks like lots of fun. That'll be my current top pick for prerelease guild. We'll see if this changes later, I suppose.
Runner-up: Gruul. I'm imagining having an aggro deck with a million gazillion combat tricks and blowing my opponents out repeatedly. Mmmm...
I don't actually have a lot to say beyond that. This format is going to be awesome.