Yeah, the ATV4 YouTube app is pretty dumbed down, and I didn't use it much on my ATV2 (and skipped the 3), but now I end up using it all the time on my ATV4. It's conspicuously the least feature-complete app I use on the ATV4.I completely agree about adding basic features like subscribe. But, it is worth noting that the ATV3 app has those things, it is the ATV4 YouTube app that is missing very basic features.
I'd like to see YouTube update the app to let long-press on a thumbnail bring up a pop-up menu like on iOS (or some sort of detail screen if necessary) for "Not interested / Add to Watch Later/ Add to playlist", and during a video, let use either another tab on the pull-down menu, or long-press, to present choices for "Thumbs up / Thumbs down / Add to playlist / Go to Channel". These are not new concepts, they're things already present in all of their other apps. And after cutting the cord, I end up watching more ads on YouTube than anywhere else (mostly on the ATV4), I would have guessed they'd want to hold on to that market.