[Thoran]

Well if you start talking about expanded shooters (like those with roleplaying elements) - Technically speaking... Oblivion is a first person shooter.

I tend to think of the first person shooter as a game that's tactically focused, minimal RP elements, perhaps some spatial puzzles. That genre of game tends to be more cinematic at the plot level. The player isn't 'planning their destiny', but rather they're a character in a developing plot that they have little control over. Half Life 2 is the best example of this - the player is swept along by titanic forces and spends his/her time fighting for survival while trying to solve the overarching puzzle. Cool stuff, just a LOT different from a roleplaying game.