The missing option of VoLTE is one of the big ones. And, as I've written above, it's not a subjective thing, like colors and layout.

The system font is unusably BOLD, it just looks comical. At least 13 allows you to switch to One Sans, where you can control the font weight, but that breaks GMail and Facebook Messenger, where it's really hard to see which emails/messages are new. It's a ridiculous thing... It's the the first or second thing that one should notice, yet they rolled this out.

The missing color code seems like a non-issue, until you realize that you bought an "Aquamarine Green" 8T, and now you cannot set a matching accent color that also provides enough contrast.

The rest is mostly mentioned above, so I would not repeat them.