Oftentimes the League of Legends community is terrible, approximately 1-2 players (on your team of 5 total) will be players who want nothing more than to:
a) play the game for 10 minutes, instead of to its completion, and will then ruin the game in order to end it quicker,
b) Intentionally feed the opposing team (which can be as a side effect of option a), which a lot of the time is purely because they dont agree with how you are playing, aka 'why use X item, when Y Item suits my playstyle better'.
c) People who will give up on a game as soon as you start losing (which given the style of game, the balance of power will shift based on the type of characters each team has, and the portion of the game you are in).
I Kid you not, in at least every second losing game (which for people who have played a fair few games, should be about half of your games) I will get somebody who complains, or leaves, or feeds the enemy team kills, or a combination of all 3. Main advantage is that the reporting system is quite simple, and if you /ignore a player, you will be unlikely to be placed with them again, which for long term players, will grant you a degree of immunity from known game-ruiners.
That said; In terms of 'DotA' style games, HoN LoL and DotA, I find the League of Legends community on a whole to be much more sympathetic, as opposed to particularly the DotA community which seems very jaded, as if losing a game is always going to be lost by somebody who isnt playing it how it is meant to be played, or a disconnect, or some such other problem (players do not seem at all optimistic that they may get a good game).
tl;dr LoL Community is on a slippery slope due to the inherent flaws of bounty based team games, and the DotA community makes the map unplayable for me except with friends (where the community becomes irrelevant).