Currently playing the demo of SWAT 4, and hoping to get the full game + addon when that comes out.
Very different from most FPS's. Right from the offset you know that this isn't a millitary game, this is a game about the police. Might not seem like much but it makes for a very different play experience. The whole emphasis of the game is saving lives, not killing, so you can forget about going in guns blazing like you would in any other FPS. Even armed perpetrators must be given a chance. Unless there is a very direct threat to you, your teammates, or a civillian, you have no authorisation to use lethal force. It can't just be the case that the suspect is holding a gun, he has to either be pointing it directly at someone, or he has to have discharged it, at which point you are authorised to use lethal force.
It makes for a very tense gameplay experience, where every room you enter is a potential deathtrap. You have to approach each situation with a caution otherwise its very easy to get yourself killed. While it's obviously been re-jigged so that it's more actioney for a gaming audience at home, it gives you an insight into just how difficult this kind of task must be.