In my personal experience playing League of Legends, negativity and abuse generally go down as you get to higher levels of play. I can go a dozen games without seeing someone rage, and when it does happen, it's rarely more than mild. Whether it's because players eventually learn that rage hurts their odds of winning, or because the toxic players are mostly weeded out, things do get better.
At the lower levels of play, however, the game is awful. Trying to get my friends fresh into the game is a crap shoot. More often than not, they are driven away by the horrendous behavior of other players. Games against the AI can help get them a little deeper into things, but eventually the bots have to be left behind to really enjoy the game.
I have high opinions of Riot despite this. They truly do go the extra mile to encourage good behavior, and I know that it's a vicious, uphill battle that they're trying to fight. I hope that other gaming companies follow their example, and perhaps someday we'll be able to clean up the gaming community as a whole.