Achievements. Over 500 little questlets to do and work on all the time, with snazzy tabards and pets as rewards? Sold. But aside from a few screenshots on Blizzard's official page, we've had little idea what it was going to look like for the players. Now, thanks to Skosiris of Wowhead, that's going to change (Wowhead's WotLK site just opened up to the public, by the way). He sent me these exclusive screenshots from the beta just a little while ago – click on for a promenade through the Achievements UI.
Gallery: Achievements UI