Sandbagging. This is something I recently noticed that I just hate. When someone clearly is skilled but chooses not to try or literally stands idle for a match and then just goes bat-shit crazy in the next round. Also when they let you get free hits. When I win or lose, I want it to be 'for real'.
Something I don't like doing is leaving a match right after I won (or after I've lost), even though I really have other things to do. It just comes off as me being rude (not that I mind when others do it to me). When I have to leave, I try to win two in a row or I leave when I've lost 2 in a row. It's difficult to show people that you aren't salty online, or an asshole waiting for the perfect win to leave.