I'm gonna go with... *drumroll*... the people. I don't think I'm exaggerating when I say that 90% of the people I've run into online (in a game or otherwise) are assholes with no respect for absolutely anyone. They're the kind of people who think: "No one here will ever find out who I am and where I live, so I won't get in trouble for being a dick. Since the only reason I ever try to act in a socially acceptable manner is out of fear of punishment, I'll be a dick, then."
Now, I understand if a kid thinks that way. But a lot of people I've met online, who clearly think like this, are adults. That's just scary. Regardless of whether they're kids or adults, they're making it impossible for me enjoy online gaming.