When you say NWN was dark, you're talking about the subject matter, not how it looked on the screen, right?
As far as straight RPGs vs. MMORPGs is concerned, I think the good thing about RPGs is that it's easy to sit down and play for half an hour and then stop. When I play NWN, I can even go online & play for a while with a buddy in a self-contained world, from what I'm told (I think you can play online in BG, also, but it seems like NWN added something to the mix, though I'm not sure what). But with MMORPGs, it's harder to play a little here and a little there. I trashed a couple of semesters of college playing MUDs (like MMORPG, only text-based), and I don't even want to get started playing anything like that.