1up is reporting, courtesy of Lead Designer Tom Chilton, that Illidan's Black Citadel will be the most challenging instance in Burning Crusade at release-- and it will have a 25-player limit. Eyonix has confirmed, so if you were planning to take 39 of your friends to a brand new raid in Burning Crusade, better disappoint a few of them.
- The PVP system will be completely revamped-- no more ladder system. Instead, you'll earn Honor Points, which can be spent for different rewards, including new gear.
- "Arena PVP," in which players can get a charter to form teams for 2v2, 3v3, or 5v5. Also, a "season-based concept" for arenas. Your guess is as good as ours on that one.
- Chilton says that putting Shamans and Pallys in both factions will let them "differentiate them more." Shaman will get Bloodlust, a level 70 party haste buff. Pallys will get a castable AOE taunt buff-- cast it on a player that's getting attacked to bring mobs to the Pally.
- Instances will have different difficulty modes at different times-- an instance played at 62 will be easier and give different rewards than the same instance at 70.