Here is a list of things that #$%$ me when playing MP as I consider them "rude". Feel free to pass comment or post your own issues - I would like to get something on a consensus from the community on what should or should not be done online.
1) The host quiting when he is loosing when they still have other team mates in the game. This is unfair as it denies the winning team a win or the "loosing" team a chance to recover.
2) Starting a game that you know will take a number of hours and being unable to complete it - I am not talking about an emergency popping up. I am talking about things like "I'll be back in 30 minutes after I have dinner" 30 minutes into a game, or "I have to go to the shops now". Don't start a game if you cannot see it through (and thereby let someone else play). If you DO start and have to leave, at least surrender so the AI does something to help your team
3) Quitting as soon as you loose you cap ship. I have played a number of games where I lost my first cap, and my second, third etc... and still won. I've won games where I have lost my homeworld. It is a team game so leaving as soon as you get a bloody nose in your first battle does not endear yourself to your team mates
4) Telling people that you are going RA at the start of a game before anyone has scouted the map etc... and then blaming them when you don't make it is not a reflection on their skill level, it is a reflection on yours. You have picked the wrong tactic since you are not isolated enough to be left alone while you tech up.
5) The game is not sim space - if you build a fleet you need to use it. Leaving your allies to be attacked (and possibly defeated) because you don't want to lose part of your fleet is poor form. And what will often happen is their fleet is defeated and then yours is - if you had stood together you would have won. Especially if you are different factions since you get abilities from two factions against the enemy...
Anyway, that is my carthartic rant for the day