Only 3 things I have a problem with.
The sniper rifle you have to aim at a enemies feet if they are far away to hit them in the head.
You can fall through or get stuck on things on lifts.
And the graphics take a while to load sometimes.

1. Ive had no real problems with sniper rifle.
2. Dont walk off of any lifts until they stop moving (especially the one at Subway that takes you to the HQ - youll end up under the map)
3. Installing 1&2 to HDD makes a world of difference, although not foolproof. But what an eyepopping feast of a game and soo fast too a marvel.