There's also the fact it had little competition from the various other time-sucking endeavors available these days (most importantly video games and movies/tv).
It's very difficult to get new gamers (from what I've seen) by trying to convince them to spend 6 hours on a game that they won't advance a level (likely) as they're so hammered with instant gratification. Heck, I played Dragon Age II for about 4-5 hours and am level 5 already.
I believe that if you're trying to get video gamers to play RPGs by competing with video games
on their terms, you'll fail every time.
If you're trying to recreate the WoW experience on a tabletop, you've already lost the battle because WoW can do it more casually, at any time, with (as you said) far more instant gratification.
So, what should D&D be trying to do, if not compete with WoW? I'd say, lure those WoW gamers in with awesome marketing and artwork, and keep them there by giving them something WoW
can't give them: a robust engine for roleplaying (and not tactical video-game style combat).
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As far as complexity, I think there is a divide between complexity and
depth that needs to be distinguished. Complexity doesn't necessarily create a deeper game experience. And, you can certainly find a very deep, very intricate game experience from very simple mechanics. See:
Apocalypse World.
As an aside: WotC may surely be concerned with maintaining the population of CharOp fanatics - yeah, they probably buy all the new power books and latest sweet DDI "build" or whatever. But, those people by and large I've found don't
play the game as much as play building characters. Is this really the population WotC should be designing for? I don't know. But, my guess is, probably not if they want to build a game that's not just going to make money, but also be an amazing roleplaying game.