As simple as that, the title is clear.
My list. Not in order.
1- Humans gaining "Kryptonian powers". It doesn't make any sense to me. See Smallville or Superman & Lois.
2- Jonathan Kent and Martha being both alive when Clark debuts as Superman. John Byrne's "Ma" and "Pa" were terrible.
3- Superboy. It doesn't make any sense. Having Superman younger and operating in Smallville. Goodybe secret identity, goodbye mystique.
4- Having Clark returning to Smallville as an adult man. Too risky and dangerous for his secret identity.
5- Kryptonian enemies. Kryptonian enemies alive, operating with full yellow sun-based powers without any training or experience. Superman must be the last son of Krypton, in my view.
6- No trunks. I miss them a lot.
7- No yellow S-Shield on the cape. I miss it. See Man of Steel (2013).
8- Superman having powers outside our galaxy. Too far-fetched. Too "powerful". I don't buy it.
9- Superman being Superman before he turns 30. I don't like that. The Donnerverse origins were 100% perfect.
10- The secret origins established by John Byrne. Not epic. Too simple. Again, Ma and Pa contributing. Nah.