The problem I seem to run into with every online competitive game I play, is the fact that humans are involved. Humans are vile, disgusting, cutthroat bastards who will fuck anyone over for a slight edge or an easy win. cheating, exploiting glitches, unsportsmanlike behavior, etc.
Most recently, I'm talking about WoW arena matches, WoW battlegrounds, Modern Warfare 2, and Battlefield: Bad Company 2. And it's not losing that bothers me. I generally win more than I lost (except in MW2, not sure why), and can still have a lot of fun during a loss. It's when someone is behaving in a way that you'd punch them in the face if they were sitting nexto you, but you can't because they're some anonymous person on the internet. That kept splitscreen gaming in check, and it's gone when playing online.
I may just have to stick to coop games for multiplayer. That way you're still enjoying a game with friends, but you don't have random douchebags spawn camping or exploiting some glitch that will take the devs a couple months to fix. I just don't understand why people can't play a game to have fun, instead of turning into assholes.