Disclaimer: I used some exaggeration from actual examples that came to mind. If anyone I play with sees this, it's not necessarily a comment on that particular situation, it's just the first thing I thought of that could've been changed around a bit to make a good example.
Above all else, roleplay! It doesn't even have to be fancy.
- The following works mechanically: "OK, I'm using Twin Strike on orc #3... do a 23 and 17 hit his AC?" "One hits" "OK, I deal... ooh! 17 damage"
- The following is much more fun and engaging for everyone involved: "I quickly fire two arrows at the orc... do a 23 and 17 hit orc #3's AC?" "One hits" "My arrow pierces through his armor, dealing... ooh! 17 damage"
Related to RPing, sitting around a table being yourself is one thing, actually playing out a realistic character is another. There's nothing wrong with playing a character similar to yourself, of course, but if you describe your character as the neurotic, paranoid type and in play you act as your typical laid-back self... why even describe the character that way?
Know your character. Related to that, don't
argue with the someone when they mention you did something wrong rules-wise. Disagreeing is fine, but don't be a jackass. Saying "You sure? Well, show it to me in the book" is much nicer than "Screw you, don't tell me how my character works!".
Don't shut down other players' ideas. Even if it sounds stupid to you. Mocking each other playfully is one thing (which almost everyone does from time to time), but telling someone "WTF, that character concept is so dumb" is another.
Know when your turn is up. If you completely forget about it, have the courtesy to delay rather than hold everyone up.
Going along with what Grygmar said, I personally have no problems with someone picking a character to round out a party,
so long as they'll have fun doing so. Don't play a Paladin if you loathe the class, but if your party needs a defender, and Paladins seem like they might be fun to play, go for it!
Don't ask to change characters
too often. This ultimately depends on your group, but some people can get irritated when you're playing a different character every other session. Understandably, there'll be times when a new book comes out and you want to play something new, or there are a couple classes you'd really like to try but haven't had a chance to. It happens. I have no issues with working in a switch of characters if someone wants to switch, but don't ask to use a Half-Elf Bard for one session, an Elf Ranger for the next, a Drow Avenger for the third, etc.
Have the courtesy to ask your DM for permission on things. If you want to use the latest cool Dragon article, ask first! If you want to play a Monster Manual race, ASK! Don't just show up to the game expecting a yes (without a backup plan), even if you're almost positive the DM won't have a problem with it... it's a matter of courtesy.
Commit to the game. If you want to play but it really doesn't fit in your schedule at all, or if you can't be bothered to get out of bed half the time, you have to consider whether or not it's hurting the group. Generally though, people are more accepting of the former than the latter
The loner character who trusts absolutely nobody is great in a single-protagonist novel or movie. RPGs are not, generally, single-protagonist forms of storytelling. Your lone-wolf-don't-talk-to-me character is more of a PITA than anything.
QFE x 1000. This I think I've run into more than anything else.
