A thing that always bugged me is the media alt text. You can't go to the media library and change it once if you got to update the text. No, you have to go to the post and update it there.
This makes sense if you need to display the image multiple times in different contexts. But why shouldn't we have the media library alt text as a single source of truth that can optionally be overwritten by the image when we specify a different alt text there? Would make more sense to me.

Other than that, WordPress should move to the next level and become more programmer friendly. You, Justin, have written tons of posts on modernizing a lot of things, and Leonardo Losoviz has excellent articles on Smashing Mag. It's really time for WordPress to adapt modern paradigms. Perhaps feature by feature, one tiny step at a single time. No need for haste, just a road map to begin with. Stay in good health, pals!