Hm...maybe it's whether or not you feel like you accomplished something? I'm insufferably smug after I get past something nasty playing Thief (ask Moose4, who had to put up with me after I took out four guards the other night), and I think that's largely because it's a well-designed game with lots of surprises. On the other hand, I played Master of Orion 3 and am thoroughly unimpressed - I've never been able to do anything other than fumble my way to a win, and it doesn't feel like you do anything other than push buttons.
And I've never been good at FPSs. I can play the Thief games because there's little to no combat involved, but twitch-gaming isn't my style. :shrug:
Yes, well, dear, you remember that your loving husband tried Planetside (online MMO first-person-shooter) for a few months and bailed because my ego couldn't take getting repeatedly getting capped by 11-year-olds. So that's why I do nice boring game things like fly Flight Simulator 2004. :)
Although with your Neverwinter Nights addiction and predisposition toward playing breakable characters, I'm STILL faintly hearing "The rage of the Red Tiger, unleashed!" in my sleep...
}:-)4