Socially, just regular politeness as far as I can tell. Some more technical things are -
- you have to play very fast, obviously, especially during war.
- estimate or negotiate the length of the game and don't drop out (it happens a lot, so much so that I think some people might phrase this as "try not to drop out" or whatever).
- use a reliable network connection (problems happen a lot) (I use a regular DSL and it seems fine, but I don't know whether it would be OK for hosting games).
- turn off combat animations.
- some people suggest low-res graphics settings, but I think that depends on your computer.
- if you need to go to the bathroom or something you'll probably have to do it while the game is running, like: play a fast turn, broadcast "brb", hit enter, run and do your thing, broadcast "back", play another fast turn and hit enter; if you were gone long enough and the turn timer ran out, you might have missed a turn.
- games in a ladder league will imply additional ettiquette, like don't drop out no matter what - but I haven't played any
- in games specified as being in the cton style, players are not supposed to diplome - but I haven't played any