I'm of the opinion things haven't changed much from their original course. Sure, there are some games that sacrifice plot and the like for uppity graphics and repetitive gameplay (CoD comes to mind); but there always have been games with poor plots like that.
The only reason there are more of these about, as far as I can see, is that the target audience has shifted. Nowadays game-playing is a lot more widespread and games like CoD are aimed at impressionable young people, who couldn't care less if the plot is bullshit: they're only playing for five minutes of distilled glory. Please don't shoot me down for raging at the yoofs; I am one myself after all.

I do disagree with their choices... but I still think there are games filling our niche too; you just have to look harder, as in the case of DF and other indie games.
I think asking so much of games like WoW is pushing it a bit. WoW has an admirably well-made plot for its size, even if they do seem to sully it by mixing in idiotic manga storylines or making stuff up purely so they can release expansions on time.
They aren't aiming solely to please RPG players: more players who'll grind or PvP lots for a tenner a month so they can get the happy feelings they crave. I don't look down on people like that; I'm just suggesting why gameplay may be a more major focus for mainstream games these days. Gaming's much more mainstream these days: you might do better comparing today's World of Warcraft to yesteryear's Dig Dug.
Nevertheless
you can actually roleplay in MMOs (I do), which is more than you can say for Baldur's Gate (sure you can play a role, but that's not actual roleplay in the DnD sense: it takes two to tango etc.).