trust me, i used to be a ****ing d2 addict, i was on at least 8 hours everyday. (mostly because there was nothing better to do.) WoW is so much better than d2, if you're going to get addicted to a game, id go with WoW.
World of Warcraft is such a terrible game, I won't even explain it, it's a generic MMORPG! Not only that, you're paying to play that? I played it for six months, and half the time I spent just running in circles, it was BORING.
As for Diablo II, everyone leaves, comes back, leaves, comes back, just take a break if you think you're playing too mcuh. I wouldn't advise quiting outright as many people return in the following months and years.
(I've just started playing again, last time I played was 2003, before that was 2000)