The biggest thing I learned from world of warcraft:<p>If you quit wow and spend all of that time that you would have been playing WoW on other tasks, you can actually accomplish some pretty amazing stuff.<p>(Get in shape, become a hacker, learn music, get girls... pretty much anything.)<p>Grinding levels in WoW takes discipline. 
Grinding levels in WoW can be a time investment similar to a second job.
You can literally waste YEARS of your life playing WoW.<p>If you have the energy and discipline to sit at a computer for 8 hours or whatever after you've done all of your daily stuff, you could just as easily be spending that time accomplishing something great or learning.<p>And you will have tangible rewards that you can take with you everywhere, not just bits on a server.