You can redo quests as often as you want. There are as of last count, over 15000 quests in the game. There are side quests and missions for every race, class, area, city, faction, skill, profession, etc. If questing is your thing, WoW is your game. Even then, if you are sad enough to complete all the quests available to you then you can start another character and do all his quests, it's ridiculous how much there is to do in that game.
Maybe I should download it and see what happens.