Some of the new features 3.1 will bring to the table include the long overdue "Dual Spec" functionality, the introduction of the terrifyingly difficult Ulduar raid, and a number of changes to every single class in the game. There's more meat on them there bones than we could ever describe in a single post -- luckily, our ninja-looting sister site WoW Insider has been tracking the update for months. We suggest studying up with their comprehensive Guide to Patch 3.1 in order to more thoroughly complain about how your class got "totally nerfed."
[Update: Hey, US-based WoWers, having trouble downloading the patch? No worries – we've got it up on the fast and the free-ious Big Download.]