Conqueror is a rune that negate itemization. Runes hold too much power/utility.
TL; DR : Runes stole roles/ broke roles by giving too much power/utility accesible to everyone.
Let's start with an example I'm personally concerned with, Tryndamere.
Tryndamere suddenly rise a lot in term of ban, what makes him so un-bearable for people ?
Well, before, even if he started to snowball, you still could play a tank and build defensive item, he then had to build last whisper, that was slowing him down from hard snowballing, as on only AD item isn't that good on Tryndamere.
But nowadays, even if you play a tank and manage to hold him down, if you don't rekt him early, he'll sooner or later counter your itemization and beat you up.
Tryndamere isn't the only champion that works this way, before only Fiora was working this way, nowadays she's barely good, because every bruiser champion stole her role by having access to Conqueror.
The same way goes for Rammus, everyone can be Rammus by running Predator, making basically picking Rammus over Olaf or Skarner a troll pick. And for Fiddlestick, who now only exist as a support because Afterschock makes him a tank, but he's not supposed to be one, so he overlaps other tank that would normally fit this role. Bard, who was a playmaker, is now some kind of burst mage bot. (Eletrocute over-buffed Thunderlord) Ezreal and Kleptomancy. Etc ...
Rune power, and especially utility power, should be toned down quite heavily in pre-season in my opinion.
Or every champion that are in trouble from "role stealing" or "role alteration" (Fiddlestick/Tryndamere) need the Mundo/Nasus/Alistar/etc... treatment, with a lot of add-up to their kit.
So either power creep everything, or tone down Runes.
One is long and hard, and is the way Riot are currently heading (1 rework per month with Nunu) And the other is just a large bunch of number change, and rune deletion/change (Kleptomancy, afterschock back to shielding, Dark harvest, etc ...)