How do you solve a problem like
Maria Aliens Colonial Marines ? You should probably start by obliterating the game code from the face of the Earth - well, it is the only way to be sure. Gearbox haven't quite gone that far, but they have issued a massive, nearly 4GB patch that fixes and tweaks a bunch of stuff, including those awful textures, that awful AI, and many more awful, awful things. It won't make the game look the way it was supposed to , but Colonial Marines should be marginally less terrible the next time you load it up on Steam.
In addition to better protecting the game's save data, fixing the Xenos in various ways, and - best of all - "[addressing] some issues that could cause improper warping for co-op players", the patch notes boast of visual improvements and a fix for Ripley's semi-sentient flamethrower bonus weapon, which "would sometimes fire continuously without player input". Now I'm no expert, but that's not something you generally want a flamethrower to do.
The patch should be on Steam now.