You couldn't go from sitting (eating/drinking) to casting. Same thing while mounted.
Mage had to talent evocation.
Mages had to put 3 points in arcane explosion to make it instant (1.5sec cast).
Windfury proccing off itself.
10 man instances.
Flight paths didn't link, so you had to choose the next flight path every time you landed.
Pots had a 3mins cooldown but could be used repeatedly during an encounter.
Resil used to reduce the chance you were crit, it wasn't changed to damage reduction until Wotlk.
PvP trinkets didn't all work the same, they were class specific.
Speaking of which, class specific items. And class specific quests.