This used to happen constantly to me as well. After removing a few command line parameters I had (now it only has -memalloc=system -nosplash -nopause), it started happening sporadically.

It can be described as follows: Character quality drops sharply (polygon count seems to drop dramatically, with player models and clothes clipping noticeably), FPS will drop to the point of the game becoming a slideshow, doors will stop showing any visible interaction, but they can still be opened and, from the player's perspective, you walk through them into untextured areas.

It sometimes fixes itself, sometimes it can be fixed by doing something that restarts the rendering engine (I turn the shadows off, then back to high), and sometimes the only solution is to restart the game.