I don't play WoW. I experienced what I am like with this sort of game when I played City of Heroes. I was obsessed enough that I kept doing it, but I was never able to keep up with any of the hard-core players (I would still be looking around or trying to figure out how to get from point A to point B) while the group I was with would already be there and done with the mission.
The problem I had with CoH was having a hero, and seeing I needed to reach uber levels in order to fly. I got Jump, then Slooooow Hover, then just stopped playing. MMO's are all pretty similar anyway, like a big dungeon crawl in D&D. Sure the NPC at the bar gave you a semblance of plot, and you can talk to folks here and there for "roleplaying", but you're not getting into detailed personalities.
Even if I did catch up, I had no concept of "roles" ala blaster, healer, whatever, I just did whatever felt fun at the time, and that made me a very sub-par player. [BTW that feeling of being stuck in a one-note role is one of the things I like least about 4E. Different thread, probably!]
Amusingly enough, a druid in WoW can spec as either a tank, damage (melee-cat), damage (ranged-blaster) or a healer. Most other classes are configured into damage, or a damage/tank, or damage/healer. "Controller" is a subset of powers, not a class.
Granted, currently you have only one "spec" and need to pay to change it (in game money), but soon "dual-spec" will allow two specs to be easy swaps. So it's actually easier to change roles in WoW than in 4e. (You'd want to change gear also in most cases, but you generally have an excess of gear, except for the top tier stuff.)
The balance between fun, time, effort ultimately led me away from games of this nature. I know I would probably be one of those obsessed/play-all-the-time guys if I let myself try WoW. And yet a lot of the time I spent doing it I would find it a joyless chore but I would keep on doing it. That's not a great situation or where I want to be with my life right now.
It's always a decision the individual makes, and there is plenty of "grinding", but not in the (from what I hear) EQ style of randomly killing boars over and over until you level. Quests generally have a semblance of plot and chain together over areas and such.
There's competition if you want to go that way, or collecting. My friends and I tend more to be the collector types. I love minipets and will spend more time getting a cute penguin than I will trying to get a new set of boots.
And of course, plenty of folks just hang around and chat with friends with a common interest, ala chat rooms/ web forums.
