The players who let time run out instead of resigning.
I don't like players who attempt to preach to me about etiquette and then do exactly what they complained about.
The players who play the London system
#1 Players who complain about a specific opening
#2 Players who leave before move 1 because opponents rating is too low
#3 Players who cheat
#4 Players who waste time by letting the clock run out instead of resigning
#5 Players who are try to disturb by trash talk, many draw and takeback offers
people who try scholar's mate when there are above 1500. its a insult ! LOL
People who mistake mouse-skills which chess knowledge. Of course we are all somewhat fast in thinking and we all see 100 premoves/s but many can not execute them physically because that requires special (non-chess) skills.
1) Players that not only are aborting a game, but let the server abort the game for them (20 seconds waste of time)
To a lesser extent:
2) Players that are trading as much pieces as possible
Discussing engine evaluations with noobs is also no good.
People who don't use the block function.
People who don't understand how provisional ratings become stable ratings.
The guys who like to spam draw offers ...