I'm of mixed feelings about it, and I think it's been covered by the replies. It's cool to see the world change as you progress through your quests. It's annoying to have your team break follow because the lead character entered a phase. And with so many quests creating a phase for you when you approach a quest location (the wolf cave in Hyjal, for example) you find your group suddenly becoming separated more often. You can work around it, but it's still a pain.