"It's important to realize that making a first-person game almost necessarily means making a game for the dedicated gamer."
...emphasis on the "almost."
I think Portal is a great example of a FPS that's interesting to serious gamers while still accessible to casual ones.
One of the main reasons I don't play more FPS titles nowadays is their length: a lot of games, like Half-Life and GTA, build these epic saga storylines and take many days to play thoroughly. In high school, I thought both of those were awesome (especially HL2 and Vice City, respectively) but now, in college, I just don't have time.
Portal is so short, I played it in one sitting. It's also simple: you have exactly one kind of gun throughout the game, and only a handful of opponents. It's the antithesis of a game like WoW, (which I realize is not FPS), and which requires a lot of in-depth game-specific knowledge and deep time commitment to become good at.
Portal goes to great lengths to teach players how to play as they go along. The developer commentary is fascinating... each of the first few levels has a specific concept that it's designed to convey. If you understand quickly, these levels go by fast. If you've never touched a controller in your life, I'd bet money that you could still do them. It's all part of the challenge.