In any given week, I'm usually playing 3-5 games from my library. On XB1, I would have to either pin all of my games, which would be ridiculous (I have a lot), cycle out pins any time I am playing a new set of games, or go to the Games and Apps section any time I play a different game. With PS4, your most recent 15 games and apps are right there, ready to go.
If I want to check messages? Achievements? For some reason, they're in the Games and Apps section. Why is Messages an app? Why is Settings an app? Why do messages automatically snap but settings don't? This isn't intuitive. On PS4, most of your typically used functionality is at the top of the UI, organized thoughtfully.
If I want to share a screenshot? You double press the home button, press Y, press the home button to go to that screenshot, wait five to ten minutes for the "Share' option to show up, and finally select where you want to share it. On PS4, you press the Share button, Upload Screenshot, and select where you want to share it.
I could give more examples, but suffice it to say the PS4 OS is organized in a thoughtful way and brings the functionality in ways that make sense. I don't like everything about the PS4 OS and it's missing some functionality, but it feels like it has a cohesive design. The XB1 OS doesn't make sense. It has a lot of functionality, but it feels cobbled together. The OS doesn't feel like it was DESIGNED. And I think the fact that they rely on you to lay it out yourself with pins to make it half-way intuitive says it all.
And, it seems like you should be able to understand this, but one can say objectively that the OS is difficult to use and unintuitive even if that person can personally make their way around it. I know how to navigate the XB1 OS, but I can still see that it's unintuitive, clunky, and poorly designed, even if it has good functionality.