I completed the campaign on Hard. The severe lack of checkpoints in the sewer level is inexcusable, especially with all the possible cheap deaths. Hop down from the level's start? Get mowed down by guns! Try and outsmart them with explosives? Nope - no grenades! Trying to move down a corridor? The enemy has infinite flashbangs! Climbing a ladder? A small platoon's waiting for you!...and your only cover can be COMPLETELY destroyed with a few bullets! Have fun!
Needless to say, I'm glad that's over. Now I've just got to get the last few single-player trophies and then it'll be time to hit the multiplayer.