For me, it's Loremaster of the old world. As a guild we'd hit the point where we've done just about everything, and most of us turned to achievement hunting to pass the time until Cataclysm. I decided I'd start chipping away at Loremaster of Eastern Kingdoms/Kalimdor. I figured I could do a little bit each day up until the December 7th release date of Cataclysm.
However now the shattering is happening tomorrow, which means a bunch of old quests are going away. If you already have your achievement, you're all set. If you don't though, if you'd done 600 quests (out of the original 700 required), you could wake up tomorrow with credit for only 400 of them.
So now the last couple of days I've been scrambling to complete the achievement (Eastern Kingdoms done, 70 to go in Kalimdor), not because I think it's more fun than it will be to do it with Old World Flying, but because I'd hate to think of not getting credit for those quests I did.
I should be able to knock it out tonight, and then chill out until Cataclysm.
Speaking of Cataclysm, if you're a new player looking to get into WoW, or an old player coming back to the game and you're in the market for a great guild, ZeGuild is always looking for cool new people. We're a fairly large, casual guild that dabbles in a little bit of everything (questing, achievements, pvp, end-game raiding, etc). Feel free to look us up!
And in other news, awesome Zeke and Death plushies!