Kind of hard to explain on paper, so provided a video link.
https://www.youtube.com/watch?v=04kEjVe3AGc&feature=youtu.be
Basically if you rotate the camera whilst crouched your character slides all over the place. Not only that, but also the player rotates around the center of the gun rather than his feet - I know this was done to fix some clipping issues but it now means players can fall off platforms, clip slightly through walls and just look generally ridiculous when rotating statically with their weapon raised.