you know ... everyone keeps going on about this and that and whatnot regarding WoW vs <game name here>
At the end of the day, after coming from GW, apart from the economy system and whatnot ... the deciding factor that blew every other game out of the water was WoW's total freedom to control the character.
Being able to run where you want, jump off whatever you want, fly where you want, ride where you want, go fishing where you want, fall-off-that-damn-aldor-lift-whenever-you-least-want , go swimming when/where you want, etc. absolutely made it for me. The fact that you could chase a hordie through an entire continent without hitting a load screen truly makes it feel like you're inside a world instead of a leveling game.
The 1st 25 levels of WoW i didn't bother with quests, I just ran around exploring every little corner of every little province (and got stuck in a tree where I had to hearth out more times than I care to admit)
The last time i felt freedom of character movement anywhere close to this was in Star Wars: Jedi Academy.