One of the most disappointing things to me has been the dashboard.
Being the same experience that I've had on my One X for years now has been a let down, and the resolution being 1080p/non- HDR was already bad on the One X, and now is just unacceptable. I was playing Destiny this afternoon, and when I open the guide, it's jarring how blurry it is. When compared with the experience that the PS5 dashboard offers, it's really, really bad.
Microsoft has been doing a very good job with it's OS. Hopefully we'll see some improvements in this area.
I get the 4K thing but aside from that, I'm appreciative of the return back into the Xbox dash (just got my Series X, been on PS5 since launch). The PS5 Ui is underdeveloped and fees like it's still in beta.