Toxicity towards off-meta players.
I play off-meta bot lane a lot (usually taking an AP champ and trying to carry in bot.) Due to champ choice people frequently accuse me of trolling before the game even begins. Because these people can't be bothered to look up statistics (where they exist on the web.) I file reports on these people as they flame especially if they did badly.
If people try to play the meta police by picking an adc, I will more than happily take a loss than trust someone else thus fighting the ADC for CS when I declare my intent to CS well in advance in champ select if prompted.
I feel like it's not my job to type a manifesto of why I want to try something.
I wish people would remember people wouldn't be at (insert rank) if they aren't relatively the same skill as you unless if you're smurfing.