I think we can all agree that certain combinations of weapons and perks result in classes in which it's incredibly easy to get kills and combines no effort or skill what so ever.
I, personally, hate people who use the Intervention. It's the most over-used gun in multiplayer, and it's incorporated into two types of "cheap" classes; quick scoping and Running-around-and-no-scoping.
Another type of player I hate is the douchebags who use Akimbo 1887's plus marathon and lightweight. Are these people just incapable of getting kills in any other way? It really pisses me off when a guy just runs up to me and blasts away.
And then there're the knife n00bs. I hope all of them burn in hell