My own personal list for the folks on mics (in no particular order):
1: Pull the mic out of your mouth. A few inches away from your face is the preferred space to make the mic sound nice and not transmit every cough, wheeze, and breath in between fist fulls of snacks.
2. Stop playing music loudly in the background. The newest Usher jam may be a kicking beat, but nobody else wants to hear it at the moment. With that said...
3. Stop singing. The quality of the mics and compression of the voice isn't great and doesn't lend itself well to those that are talented singers. And more than likely you're not a talented singer in the first place.
4. If your mic has a mute button use it when you're not talking. Hearing "That's fucking bullshit!" every time you die doesn't make your team want to rush to your side and save you.
5. If you win big or lose badly, show some class. If you win, try telling some of the other people thanks for a good game or thank your team for their help. If you lose, don't scream at yell at everyone and accuse them of cheating.
6. Don't be racist/homophobic/sexist. Seriously, does this need explaining?
7. Threatening to beat up someone in real life isn't frightening, it's just funny and sad. Yes, you are very tough and we're sure you beat up plenty of people every day for their lunch money. We'll just sit back and laugh at you while you show off what an internet tough guy you are.
8. This is a public game and not your personal chat room. We don't care if you and your friend are totally going to get high later or what class is going to suck tomorrow. You can start a private lobby for that and not be interrupted by people playing a pesky game.
I'm sure I've missed a few, but these are the ones that always annoy me.
*edited because I can't spell "mute" apparently