You can totally switch it to first person mode!
By default it's top-down, so there are no textures for the top of the maps, but it really changes the feel of the game. I wouldn't recommend playing this in a random stranger game though. You lose the tactical view and will probably die a lot.
Still pretty fun.
Found this at http://www.ripten.com/2010/07/21/play-alien-swarm-in-first-person-sam-n/
(Taken from the site)
Alien Swarm, the delightful and free top-down shooter from Valve that we reviewed yesterday, is playable in first-person mode via a few entries in the game’s developer console. If you were put off by the top-down viewpoint, perhaps this could entice you in.
Shrimpsess on the Steam forums has put up a list of useful console commands for the game, including the method of playing in first-person. Here’s the relevant info:
First one enables firstperson mode, second one hides your marine so you can see, third one changes the camera controls to be more FPS like.
To enable the console, go to Options > Keyboard > Enable developer console. Then hit the “`” key on the keyboard (~ in the US) – the one to the left of the “1″ key in the top left. Then enter each of the above commands to set first-person mode on.
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