Arcade mode balance changes
This is (possibly) one of the most trivial things to be annoyed about, but the balance changes done in ARAM and in the upcoming URF are the complete opposite of what those modes are supposed to be about. The reason these game modes don't have a ranked ladder is the same as why they shouldn't have balance changes; you are playing for fun. Sometimes you get something strong, sometimes you don't. I do remember a lot of people complaining that champions like Lux, Malza or Ziggs were obnoxious to play against in ARAM but that's why everyone starts with extra MR. Similarly with URF, a lot of people used to complain that Kassa, Sona and, later, Garen were in every single game/too strong, but... so what?
The point of these modes was always that it's unbalanced. URF's entire appeal is the fact that you can break the game, what's the point of balancing a broken game just so something like Caitlyn stands a chance against something like Kassadin or Fizz? ARAM is similar (though to a lesser extent). And even then, if you're going to target overpowered champions, target their abilities, not their damage. For example, it doesn't matter if Shaco or Fiddlesticks deal 10% of their normal damage, what made them extremely strong in ARAM was always in their kit.
Finally, in Riot's defense, URF is their creation and they should do what they want with it, but ARAM is something the community came up with. "Alright, so jump back in a custom game and play ARAM", someone might say. Well, this really isn't possible anymore because there are very few if any custom lobbies dedicated to ARAM these days because most people won't bother, while others straight up don't know ARAM balance changes were even a thing. Riot made a lot of players happy by creating a dedicated ARAM mode, only to split that same playerbase and making random ARAM lobbies impossible to co-exist in the process.
I know it isn't possible to listen to everyone's concerns and take them into account, and I also know this isn't reddit so many of these concerns fall on deaf ears, but this has been bothering me for a while so I thought I'd share what I think, for whoever cares enough to read it.