I played a very annoying game last night, and noticed some very frustrating problems:
It was an open campaign game with three randoms. One of those, God curse his soul, did nothing but run ahead, leaving enemies standing left and right, and either went down too far ahead to save him, or, even more annoyingly, forced a teleport for all of the rest of us, totally screwing with our game experience. I really don't like the whole teleporting mechanic, as one player can make decisions for everybody else. Instead, the teleport should only happen to players that are too far away (ahead or behind) from the majority of other players. There could also be something like an "automatic death zone" (like in Bulletstorm Echoes) if you get too far ahead and away from the other players. Also, teleporting should only happen when at least two players have reached a certain area to speed up the game, and, at the very least, should give you a 10 second warning, to give you some more time to grab some ammo.
Similarly, the same 10 second warning should happen before you leave your current location through a door/cutscene, like "leaving area in 10-9... seconds".
In the same spirit, on another occasion, my first time playing the game, I joined a public campaign, where, unfortunately, the host skipped all the cutscenes. I would much prefer a system like in Halo Reach or Lost Planet 2, where every player has to vote to skip a cutscene, and the majority wins.
And lastly, I have an issue with the forced resets of weapon loadouts at specific points (Chapters, Cutscenes, Acts). I lose cool weapons and ammo, just because I go through a door? Why, exactly? Yes, you should never end up with no ammo at the start of a new chapter, but why take away your surplus ammo, or your special weapon. I found that quite annoying.
And before anyone complains that I'm just complaining, would I bother if I didn't really like it?