I always find myself playing the game in bursts. I enjoy the game for the most part, except ZED time I hate you!, but I figured out why I usually stop playing the game. It has to do with people who don't understand how to play as a team or are just utter noobs. 
 I have beaten some of the maps on hell on earth and suicidal difficulties but I right now I'm trying to do all the maps on normal long and then progressing up just because that's how I am but I may have to skip that because usually normal mode attracts the idiots. 
 
So I'm doing Westlondon and I get in on Wave 2 with a full team and it turns out they're using the strategy of hiding behind the big metal gate by the police station. I notice that they aren't closing the back doors though and they actually almost all died because they didn't weld the metal door and it just kept opening and they kept mashing E, or whatever they have mapped, to close the door rather than shooting whats hitting them. 
I get in and I lock the two back doors and then the front one and I advise everyone to stand up front as to avoid the enemies possibly coming from behind. 
Our group make up has 3 Support guys, a Sharpshooter, a Commando and Medic. 
The Medic doesn't have an MP7 and has a flamethrower for some reason. 
The other support guys and the shooter are just fine. 
but the commando is completely tard, guy would run up to the gate and he would throw a grenade and he'd kill himself and claim he didn't throw the grenade when we can clearly see that it said he blew up. So he drops his SCAR and someone picks it up and then a river of bitching about getting it back ensues even though he is dead and there are still 80 enemies in the wave left. So finally he gets his stupid gun back and the next wave we go back and guess what he does. He kills himself again. So next wave he decides he is going to switch to medic. Why? Who knows not like you need two. So at least he grabs himself an mp7. The thing about this though is it turns out he doesn't know how to heal with it, and we explain that he has to use his alternate fire for it (usually it's middle mouse) and he can't comprehend it so he bitches about not knowing how to use it. Finally he learns how to use it and then manages to kill himself once again. 
 
In the end we end up finishing it out, but golly we shouldn't have. I wanted to slap myself for being in that game.