Having posted this picture, I have to vouch for COD4.
Death by bouncing Betty is the shittiest, most infuriating way to be killed.
In COD4, the claymores have lasers coming from them that make it impossible for someone to plant them right behind a doorframe without a cautious enemy having a chance to spot them. In WAW, Betties are cheap-ass proximity mines that can be completely hidden without a trace and can kill even the most cautious player.
Also, SMGs control the fucking battlefield in WAW. MP40s and PPSh-41s can obliterate anybody, at any range. Like sniping? Make your first shot count, because the guy you just cracked a shot at can blow your head off from across the map if he's sporting an MP40.
Also, clipping issues ahoy! A guy in the window of a house can crouch and see the ground directly in front of him with only the very top of his head peaking out. Oh, and he can shoot you, too.
...and don't get me started on those tard-ass tanks. Call of Duty's multiplayer has always been based around extremely tight infantry combat. Throwing vehicles into the mix sends everything straight to hell.