Just in line with some of the other posts, I felt like chipping in that I'm really curious to the degradation of productivity & motivation that occurs in life after quitting WoW.
It's fascinating to think that less productive, less useful things tend to fill in the gaps allowed without WoW. I've played since vanilla and have "quit" a handful of times from between a few months and a year, and rather then see a sudden increase in grades or social activity, I actually saw a decrease in quarter GPA & went out even less. Time spent in WoW was replaced with books or miscellaneous television (Netflix). All the time was dumped into similarly useless hobbies- such as learning to code (albeit, a badass) game of tic-tac-toe, fix various electronic devices, and even picky things about running\exercise techniques. All in all, things that I haven't used since I was specifically focusing on them.
The concept of a challenge is interesting. I think I actually have given that as a reason to my parents for playing- it's the only thing I do that requires a full use of my brain. I played sports through the beginning of high school, but it didn't get much more complicated then avoid people, run fast, practice repetitive motions. I play multiple instruments, but after the initial learning curve it's pretty mindless, also. Occasionally I have to apply myself for homework, but that's an occurrence so rare it's actually pleasant. More competitive premades or arenas are the only thing I've really seen where you actually have to gain mastery over what you learn, and then apply it to multiple situations in multiple different ways and all of it is subject to change depending on what your opponents are doing - and after gaining that point of mastery, it comes to predicting your opponents move and setting up the fight multiple moves ahead. Sort of like an elaborate game of chess, that is why I enjoy WoW - it can be as complex as you want it to.
The other side of things is that if I'm not sitting in the living room facing everyone while playing WoW, I'm going to be sitting in the living room reading a book ignoring everyone or watching television with the family, either activity seems to be just as antisocial as the game itself. At least in the game, I think, you're using your brain - rather then just rotting your brain away on television.
It's all fairly intriguing, I think.