My main complaint is that there isn’t a consistent experience between apps. Things are in different places, actions do different things in different apps, no gesture support, subpar players with horrible scrubbing, etc.
I think all apps should be held to a few basic UI/UX standards so going from one app to another isn’t a completely different experience.
The attached photo is just one example. Within the Apple TV app itself, long pressing an icon brings up this great menu allowing you to go straight to the show or the episode page. Long pressing in most other apps removes the show from your now playing list! All of these conflicting actions need to be rectified.
If they make it unified like CarPlay they will have the same lack of Apps as CarPlay.
Keep in mind CarPlay also has legal limitations due to being a screen in a car. So most companies have a vested interest in following the guidelines given by Apple as it saves them from having issues.