Standard major patch day. Anyone who's been around longer than just Wrath knows how it works.
As far as what's in 4.0: Class changes, 31-point talent trees, NO world changes, no new content, items are changed, stats are changed, UI changes for Cata, no new race/class combos, no new quests/leveling. New questing/leveling is tied to the new map, which comes with the next pre-Cata patch.
4.0.3 (probably next month sometime) will bring everything that's in beta except the 80-85+endgame content and the new races. Then those last things are added on Dec 7th with Cata release.