**I'm playing on the PS3 here; none of this is in reference to 360 or PC**
The first (and worst) of these issues which I would like to bring to your attention is hit detection. There have been countless instances where I have emptied entire clips of my shotgun into a player from point-blank range, or bashed them 4 or 5 times in the back, only to have them turn around and kill me instantly. I have been shown Kill Replays of "headshots" that were several feet over my head. Multiple times I have been shot or bashed to death by someone who was not even facing me; I can literally be looking at their back when I die, and the Kill Replay shows them shooting/bashing thin air, yet I still fall. There have even been times when I drop dead for no apparent reason, and a kill replay is not even shown (suspicious) - although this, I am willing to give you the benefit of the doubt and write off as a stray bullet coming through a wall or an explosive going off without making a sound (suspicious as well). And I have also experienced the yin to this yang, where I've come out of fights thinking, "There's no way I should have been able to kill that guy, I should be dead." Most of these occur when I'm not experiencing any sort of lag or connection problems.
Next issue: are weapons supposed to be able to get jammed? If they are, that is fine, but if not, that would definitely be a problem because I have been killed several times because my fully loaded weapon doesn't fire. Has this happened to anyone else?
Number three: In one of the maps (I think it's the "Wall Street" one) at the top floor of the main building, overlooking the intersection, there is a big hole in the wall from some sort of crashed alien pod that seems to be a perfect spot for snipers (I hope this is descriptive enough). Well, it is perfect except for the fact that whenever I try to aim down the sights, the scope and barrel don't show up and I can't see where I'm aiming. I don't know if this is a glitch or if it was designed like this; neither would surprise me actually, it's almost too perfect...
Finally, I think the power dropoff in the range for the shotgun is too steep; if the player is more than a few meters in front of you, the gun is useless. I'll admit that this is nitpicky but I just wanted to point that out.
Anyway, those are the big issues I've some problems with, but other than that, I've enjoyed the game immensely. It just seems that the multiplayer portion wasn't given nearly the amount of attention that the single-player portion was, although I can't complain too much about that, the campaign was the best I've played in nearly a decade - good work!