I wonder if the level of comfort one would have with the UI depends on how comfortable one is with mentally "context switching".
A lot of the "why is everything an app!" comments make me think of that. A lot of the options don't really take much longer, and still depend on an internet connection just like on the 360, but since the guide menu on the 360 was a smaller menu that still had your game showing behind it, I think some people feel more comfortable with that. You still had to hit buttons to "load" your friends, or "load" the party interface, but since there was the game sitting quietly and faded out in the background, it was easier for some to keep track of. There was always only ever 1 "main" thing to keep in mind, and that was your game or your dashboard.
Now with the Xbox One, a lot of things you load into now are fullscreen. Even though the game is still running in the background as before (or on the side in the case of snap), the action of going fullscreen into something else, or seeing two things side by side seems to throw people off, because you can mentally lose track of "home". In addition since "home" may not actually be "home" now (the dashboard is now just another layer on the stack), you get the "why does mashing B on the home screen taking me back into a game??" complaints. On the 360, mashing B would either taking you back to your game, or back to the top level of the app, because the 360 could only ever run 1 thing at a time. So it was more restricted in that sense, but more "comfortable" because it only kept the user thinking about 1 thing at a time, and there was always a "home" to go back to. Xbox One can run 4 different apps, and also keep track of your entire history of loading stuff, so even though it's cool that it can do this, this also means more for the user to mentally track.
It also makes me think of a lot of the Windows 8 issues people were having. A full screen start menu still effectively worked the same as the old start menu, functionality wise, but a lot of the complaints were "why is this throwing me into some full screen menu". The feeling was that it was taking you to some completely different place, which a lot of people don't like, and the screen covering up your desktop made one lose track of "home".