Because instead of static animations in combat, stand around combat, boring text for quests, you have a combat system where you are constantly moving, actively dodging, when you run away someone's attack doesn't automatically hit you, and quests are replaced by events that just happen. You don't ever need to go up to someone with a quest marker, you just walk into a village and its under attack. And you save it. If someone else walks into that village a week later, it is still saved. If you fail to save it, then it is still destroyed a week later, and probably infested with bandits that other people have to drive out.
Everyone gets all the exp and loot from any monster they help kill.... I could go on, just check out "Ambassador's Mission GW2" on youtube.
I've played GW2, it is awesome. Trying to play another MMO, like Rift, is super boring in comparison.