Huge Question.

Alright, well let's take a look at what has changed since pre season.

Runes have been heavily rebalanced and now EVERY character in the game has multiple viable options for runes depending on your playstyle and what you want. Certain runes are better for a general scenario without completely breaking every other runes viability. Fleet footwork on ADCs for example is generally the most optimal, but if you want to be agressive PTA is still a very good choice that is optimal for several ADCs and fleet footwork is showing up in games where you won't get pressured.

Riven is able to comfortably run Conqueror, Unsealed spellbook, and Aftershock - 3 entirely different trees - based off of what the match ups are. Sejuani is running both Aftershock and Fleet footwork depending on the needs for the match. Gangplank who originally only had Kleptomancy is now starting to run grasp more often, and we have enchanters making decisions between Aery for more balanced runes, comet if they want to optimize lane pressure, and some enchanters running things like Guardian and Spellbook as well. So runes are much better than they were and you have options there.

People didn't like how Stopwatch was blocking out AD assassins as the class felt bad on the whole. Stopwatch has been nerfed in sell price and active timing, duskblade has been nerfed and compensation given to EVERY builder of it so they have more choices, and tabi was nerfed so you could get more out of the lethality.

Tanks were super strong and everyone was annoyed by it. Now we have conqueror to deal with it and top lane is much more fighter centric in solo queue.

Guinsoo has been altered to be a more viable item on more characters and we are starting to see more melee friendly mechanics that ranged cannot abuse as easily.

New runes like Chrysalis and Bone plate have been added if you want to have better odds at surviving, but people want to do max damage so they ignore how valuable those two runes are for survival.

TL;DR, runes have been massively altered to be more viable for different roles and characters, items have had massive alterations to bring back underwhelming classes, we have a much wider viable selection of characters, the meta has shifted massively since pre season.