The only frustrating thing about the way Riot balances (and it's only getting worse), is how certain champions just start completely overshadowing each other.
I mean think about it:
At one point, Kat was literally one of the best heroes in the game. She was the best laner. She infinite harass with shunpo and bouncing blades. No mana was a big deal back then. Her ult was pretty damn strong too.
However, as more heroes were added, she became overshadowed. There were better assassins and better laners. Whether or not you agree with me, Miss Fortune was superior to her in almost every way. Akali was a better assassin, and had the similar "ult cooldown" mechanic after getting kills or assists.
Akali was actually really popular for awhile. And while she doesn't have the exact same role as Kat, there were a lot of similarities. After the Kat buff, Akali isn't used anymore. They basically gave her everything Akali had and more!
She gained Akali's Mark of the Assassin skill (LOL). She got something similar to Akali's Crescent Slash with her new W (Sinister Steel). Sinister Steel is actually much better than Crescent Slash however, doing more damage and gaining speed if it hits. Her Shunpo is similar to Akali's ult, except in some ways better since it gives the damage reduction passively now, which makes it much safer to initiate with, especially with her ultimate. Her ultimate btw now gives the reduced healing debuff to 3 targets, which is huge.
And guess what? You never see Akali anymore. Ironically, you never see Miss Fortune anymore either.
Now you guys may say that these heroes actually have nothing in common, and that this is all just coincidence, but there is definitely a pattern here. Riot is running out of ideas, and instead of creating something new, they have to simply rehash old abilities on new champions or remakes. This is partly because they release so fast, but partly because they're not willing to add any "anti-fun" mechanics to the game.
Consider Rubick from DotA. His ultimate is awesome, he can steal any spell in the game from an enemy and use it against them for a short time. LoL will never have this. Too many champions being added and too many abilities to fool with. In fact it seems like the scope of spells and abilities Riot will add seems rather small. How many different ways can you do a Garen ult? How many different ways can we do the 1-hero-2-forms concept (Nidalee, Jayce, Elise, etc.)? How many different temporary invis ults can we add (Vayne, Talon, Rengar, etc.)?
They're not willing to add crazy game-breaking mechanics like DotA, which are fresh and completely different than anything that has been done before (such as a Champion which teleports mapwide for 12 seconds and takes an ally with him), and ironically are adding much faster than DotA is now (LoL has only been out for 3 years, and they've surpassed the number of heroes in a game that's been out for over 10).
From a balance standpoint, this is a nightmare. In addition, it forces people to play the heroes that are currently "good". Kat was good for a long time, then sucked (not because she was nerfed, because the new heroes were just plain better) and faded into obscurity, then got buffed and is good again.
I don't like that you can get attached to a hero, then that hero becomes subpar until they finally buff it again. In DotA, every hero is good, they don't balance through obscurity like that, so if you get attached to someone you can keep using them without fear of them becoming obsolete one day.
In the end it's just personal preference I guess, it just bothers me to think that the game revolves around "flavor of the patch" balance, instead of an overall and intricate measure of each champion's strengths and weaknesses to one another.