I don't like %HP mechanics
As the title says.
I mean, I understand what it's for. It gives a champion a relatively fixed power level on an ability regardless of target, and helps reduce the whole "feast or famine" problem that champions have. But... it's really annoying to play with and against. In a game where gold - and the items that gold buys - matters so much, it's pretty frustrating to have a widespread mechanic that massively reduces the effectiveness of my item choices.
Put simply, I don't like it that regardless of how successful I am, a champion can simply push a button and do the same portion of my life. I don't like that abilities cost the same portion of my health regardless of how much I have, because it renders my upgrades less meaningful.
It would be less frustrating if the prevalence of %HP mechanics didn't make building health actively bad in some cases. Take Soraka, for example: It's one thing to have stats that are useless, but it takes a huge chunk of the pleasure of picking up an upgrade when you know that upgrade is wrecking your efficiency... and that you need the upgrade, because it's mandatory for your role as a support. She's not the only one, either. Mundo, Vayne, Kindred, Blade of the Ruined King... there are a ton of items and enemies that do %HP damage.
I'm not sure there's a really good solution. Maybe reducing the difference in maximum health between tanks and non-tanks to prevent burst abilities from being nonthreatening to tanks and lethal to squishies. Maybe having health regeneration scale with maximum health innately in some way. I don't know. I just know it's not fun buying an item and not only seeing no improvement, but instead seeing my power go down because of the prevalence of %HP mechanics.