Player collision since OFP is big problem, it can be an immersion breaker, or mission breaker if it makes you killed when you get stuck or your character is standing by himself. In multiplayer it can be used by exploiters and break any honest and fair public TvT session, ruining players experiences.

I tried to sum all the major issues of the character collision in this video :

http://youtu.be/ZTgGf1nIxw0

1. Player can cross a lot of walls (maybe 15% of Stratis objects).
2. Shoot through concrete.
3. See through wall.
4. Be stuck under building floors.
5. Unable to shoot low when the upper body is on a edge.
6. Have its character standing itself uncover when crawling.

The character collision deserve more love.