Do not assume you are carrying the team, it might feel like it, but you're not.
Do not get uppity and snotty. No one cares how many 50s you have or if you've played since beta. I've played since I1 and have a 50 and even I don't care about it.
Do not become so inflexible that you ruin your own game experience. Everybody has their own definition of "correct" tactics, but sometimes going with the flow of the team works out just fine and adds some excitement.
Do not turn down every blind invite. Are a lot of PUGs a waste of time? Sure, but a lot of them are very fun too. It doesn't take long to see which way a team is headed.
Do not put together a team and then say, "what do you guys want to do?"
Do not use the broadcast channel for your personal chat room. Man, that's annoying.