I've played this game for about 9 years and regret the amount of time I've wasted in it.
Things that bother me about it:
-doesn't have a good sense of balance (supposedly kept in line with micro-adjustments on champs that seem over or under powered, but always feels off the mark)
-community is not great (almost every game gets someone flaming)
-patches seem to make the game dumber and simpler
-matches get decided in first half the game, and the second half is just the inevitable long push of waiting for one of the teams to finish
-there's over 150 champions to choose from, but the new ones just don't feel on the same level as the old ones, resulting in them getting banned each game and old ones feeling extremely boring by comparison