I have been playing since nearly the release of WoW as many have, but I have to say I am fairly disappointed.
The quests in Northrend are a lot more fun than that of Burning Crusade and to me, of even the quests of the original game's quests. They are well designed to keep the player on track. The environment is very impressive as well. I have not yet seen a zone I did not enjoy exploring and questing in. As well as the new environments, the new 5 man instanced dungeons are exceptionally well done. Not too much trash, get straight to the bosses which are fun to fight. New pvp system will be very enjoyable compared to before. Many casual players will be happy.
The bad.
The lack of group quests in Northrend completely tears away the feeling of teamwork and cooperation from the game. I have never felt so alone while questing as I do now, even though I am surrounded by thousands. The dungeons do not give much experience at all, so unless you have quests going in, there really is no point to doing them more than maybe once or twice. The raids were designed to be by far the easiest up to date and provide absolutely no challenge. Overall, this game provides no challenge and nothing new to experience besides the pvp aspect.
Game is fun, but for how long? That is the question.