Here are a few thoughts I have on playing MP
1) Having played SP for ages until I obtained a retail version, MP is alot more demanding, but I am finding it more fun due to the human interaction.
2) You need to be prepared to loose - especially is you are new. It takes time to learn. But you will learn more playing an MP game then SP I think ONCE YOU HAVE LEARNED THE GAME BASICS IN SP FIRST!!! A lot of people are being put off by being whipped online the first day they own the game. You can't expect a driver who just got their license to be good in a race - so you need to practice some first.
3) You need to ensure teams are balanced before starting to it doesn't become a stomp session. It is OK to ask for the teams to be changed, or to leave to find another game - jut be polite about it. Instead of a 3v3,4v4,5v5, suggest 2v4, 3v5 etc... This will make things interesting for everyone since the good players are challenged more and the new players can get extra support.
4) Some trash talking can add to the fun, but respect still needs to be maintained. Generally though I find much of the chatter is ally based, as there is no time for a major chin way with the opposition. The lobby/pre-game screen are good places to ask questions while people get ready to start a game.
5) People who quit because they think they are loosing do piss the other players off (myself included sometimes) since they have commited to a game - if you cannot afford to play for 2-3 hours, play SP or come back when you do have time. That way everyone is happy and you still get your game (of course generally we would prefer to have you online playing as the more the merrier).
6) Respect the game mode being setup by the host - they are hosting so you have chosen to play THEIR game. I realise alot of people do not know how to configure their setup to host, so it is in everyone's interest not to piss a host off. I have not gotten to the stage of banning people yet, but I am getting fed up with people who quit once their cap is destroyed even though they have a good empire and are being targetted by someone who pumped everything into some ships at the start and then rushed them. If you survive that with allies, you are often stronger then they are.
7) The lobby is not a dating service - asking if their are hot guys around is NOT what the lobby is for!!!
8) Turn off downloads and CPU intensive applications - it is not fair to others if the game is constantly lagging due to you reserving resouces for other purposes. If you want to play the game online, do your utmost to make it good for all - I personally do not like dropping people, but I would rather do that then loose everyone else who is in the game.
The biggest thing to remember is to RESPECT the other people online with you and of course to remember that it IS just a game, even if you do really want to win