I haven't had the chance to play too much of the series, but I've kinda followed the storyline of the first two between reviews and whatnot. The third is actually sounds really interesting. They're putting a much larger emphasis on co-op play, similar to L4D in the sense that the game is meant to be experienced with two players.
The interesting aspect is that the cooperative players have conflicting goals. Since the second player has drastically different physical properties, he can reach/see things the first player can't. Traditionally in co-op games, if you see something to give your partner a hand you do it right away. But in FEAR 3, since your end desires differ, you may chose not to help one another everytime. In fact, there has been some hinting around that the two may conflict at times, perhaps changing the mode to a versus on the spot. Can you imagine the possibilites as far a alternate routes and endings go?