WoW is not crack, but it is very fun. You just need to go in knowing that you are playing for part-time fun and enjoy the game without needing to be the biggest badass around.
If you're the guy who wants to reach the highest-level, have the coolest toys in the game, and reach the end of the end, then don't get involved. That goal is only reached by playing ALL THE TIME, and even then you will need lots of help from other players. This approach is completely inconsistent with productive membership in society, nevermind premed classes. (As a corollary, if you go that route you will soon come to the sad realization that your level-70 pals with unbelievable stats and gold are actually 13 years old.)
As premedmind said, the game is a masterpiece. There are plenty of really fun things to do (exploring areas, trying out different character classes, reaching early levels). It is big enough, with enough other players of different interests and levels, that you can play forever without getting sucked into the truly time-intensive content. I never grouped with anyone else (because then I wouldn't be able to leave when I wanted) and I never had a problem finding fun stuff to do. It *is* tough to play for just an hour, but it's possible, and the promise of a longer session (once that orgo problem set is finished) is a great motivator.
I played throughout premed studies (though I took a hiatus for the month before the MCAT) and was fine. So if you go in knowing what you're there for, set limits, and enjoy yourself, you'll be fine.
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