Just some things off the top of my head
Marathon, Lightweight, Commando combined into a clusterfuck of nooby knifing
One Man Army Noob Tubes and Danger Close
Getting shot despite being 20 feet past the corner I supposedly die at
Firing an entire clip of UMP 45 into someones face (should be 3 shot kill) and not getting the kill
Getting killed by a shotgun across the map
Claymore planting, corner hiding, shotgun weilding, campers
People who that that Domination is team deathmatch
Overpowered Killstreaks
The fact that 1 round from a 50. Cal wont always kill but a knife will
Crappy maps compared to COD 4
People who somehow think that quick scoping takes skill (are they not aware of the heavy aim assist?)