TBC
Flying mounts
New continent
Accessible 10-man raid with epic gear
Arena
Overarching storyline
Heroics
Socketed items
New races
All specs viable for PVE
WOTLK
Vehicular combat
Interesting heroics
New class
BOA Items
Hair studio
10-man/25-man versions of raids
Achievements
Fort mechanics (WG)
Phasing
Cataclysm
Archaeology <- LOL