I've been playing MMOs since EQ originally came out and there's reasons WoW is so popular:
1) It has the Warcraft name and it's made by Blizzard. This means that mom and dad instantly are ok with it.
2) It piggybacked off of Diablo. Diablo and D2 were insanely popular games and WoW is the next "step-up".
3) It's acessable and simple to play and you level relatively quickly. My 56y.o. dad can play WoW with my brother and I and he does well. He never hit a single target in Counter Strike, Battlefield1942 made him motion sick, Rome: Total War took too much thought and strategy. But WoW? Now that's a game he can sit down and grind an hour or so per night and just plain have fun. The interface is easy, you don't have to micromanage skills (like EQ), you don't lose anything when you die (EQ and Lineage) and the end game involves more than just PvP (DAoC).
4) Everyone's doin it. I know it sounds cheesy but I play WoW with people I went to highschool with, my dad, my brother, my friends at college etc. We all hop on vent and shoot the breeze just like we used to and it's fun.
5) The PvP aspect. Nothing relieves stress as well as killing some alliance player that you don't know. Your peen grows about 10" instantly and you forgot that your wife is sleeping with your brother. Why? Because you know the other guy is screaming and beating his keyboard and crying about how overpowered your class is. For the people that die it gives them the drama that they are not getting in their every dyay life. Why are the ATOT boards so hostile? Why do every YAGT posters get chewed to bits? Because people lead generally boring lives and chewing out some person on the forums makes them feel better about themselves in a safe way...same way with WoW.
The biggest imho:
6) It's a giant treasure hunt. Every MMO is one grind after another, what else is there to do? But what makes WoW (and made Diablo2) so popular is killing super monsters to get a sweet drop. You play for weeks and eventually wonder why the heck you're even playing any more. You say "One last mob"...."ok, 1 more till I quit"...."Ok I swear this is the last one"...."bah forget it...well, one more" then BANG the Krol Blade drops from the next mob and you have reason to keep playing for a month.