As far as gameplay is concerned, I'd say there's none.
As for other aspects, IMO some of these "guidelines" for online etiquette would be:
1.- No ragequitting (lol)
2.- No mic abuse (gawd...)
3.- Take the income of battles normally. If you lost, well, don't go all anal about it and start sending hatemail... and if you win, don't act like "lololol i won u noob" or something like that.
I'd also include lag switching, but that's kinda hard to tell imo.