I love me some single player. I like co-op. However, while there are many mistakes SP games can make, when co-op goes bad, it really goes bad. This happens in various ways.
To many players- Co-op can become a real mess when there to many players. Lets face it, Sven Co-op, 16 player mod for Left 4 Dead 2, all bad. People keep blocking each others shots, and everyone has to race eachother for some action. Failing to do so means you have to watch a dozen others have all the fun. It's like spectator mode.
No death penalty. Aka. Spawn Rush Die- Spawn Rush Die is a big problem and a pet peeve of mine. Common in the early days of co-op such as Doom, and still present in games today. This is when death simply means respawning. The result is players who run right at the enemies with no thought and die, knowing they will respawn with full health and ammo. If one does it, the lack of support means others will die, causing them to addopt this stratagy, and so it becomes contagous.
Levels not made for co-op- Similar, and sometimes the cause of the above 2. When a level isn't made for co-op it can end up with a game that feels like riding a crowded train even with 2. Sometimes even getting locked out of a vital area or stuck in scenery.
Pickups- This is one that plauges internet play with strangers. It can be summed up like this.
"Look at all the guns and items. Theres enough here for a team of 4! ... Mine!"
Sometimes this can be accidental. In one game I remember this.
"Sorry dude. They were to close together and I lagged. I would drop some for you if I could."
and finally what I like to call
"The spare knob"- This is where at least one player, for whatever reason, feels useless, and maybe even a burdon to the team. Maybe they keep getting sniped and have to be revived constantly. Maybe lag causes them to burn through ammo without hitting anything. Perhaps there class isn't really needed here. Either way, when one player feels like he's not paticapating, it can bring everyone down, or at least I feel it does.