most mmo's have promised this but almost none of them deliver on it, and for good reason. let's think about this for a second.
let's say you have a user base of 2 million people, not that unusual. at the starting hub, there's probably 500,000 people if this is the first weekend the game is live. if this is the case, only one or two groups can do a quest at a time, because their result will change what can and can't be done for the groups behind them.
what does this mean? the game must have thousands and thousands of quests lined up, along with variations. unless it changes something that is truly unimportant (like whether the trees have leaves on them or not) it's pretty much impossible to implement.