My thoughts:
DISCLOSURE: I am playing with the Kinect unplugged.
I like the UI. I think it has potential. It's certainly better than the "new Xbox 360 experience" which I abhor. But there's a lot of work to be done. Settings is too buried in my opinion. Getting to the store feels distant as well.
With Kinect unplugged, you manually have to sign back into Xbox Live each time you turn the console on. This is unacceptable.
Is it me, or have features been removed when inviting people to games? I challenged a friend to a game of KI and we actually had to call each other on the phone to figure out how to complete the process. I'm still not 100% clear on it. For example, on 360: When friends are playing a particular game, I would get a notification: "So-and-so is playing Gears of War". This apparently doesn't happen on Xbox One.
I like the activity feed.
2 apps, at least on iOS: a 360 app and a Xbox One app...WHY? Microsoft, of all people -- a software company -- should know better. It's silly to splinter the experience in such a way. Also, I find the Xbox One iOS app a complete mess. The app "language" is completely counter-intuitive to almost every other iOS app. Swiping left or right doesn't delete emails, menus are multiple levels deep when they don't need to be. Information is conveyed in a way that simply doesn't make sense (On the homescreen clicking into a game in "Recent" doesn't show you who is playing that game, your achievements, etc...nothing relevant.)
The controller feels "pin-faced" or "mushed" or something...Like all of the buttons are trying to converged at an apex and sinking into a hole. For my huge hands, this is a step back for me.
The bumpers suck. I'll get used to them but now I have to lift my finger to activate them, where previously they rested on top in a "natural" manner.
The console doesn't look as bad as I anticipated when it is resting in my entertainment center. When I heaved the Xbox One box out of the Amazon packaging, I anticipated the worst. It's no PS4 in regards to design, but it does a good job of shutting the fuck up and sitting in its designated spot.
The fan in the powerbrick is ridiculous. In fact, scratch that -- the powerbrick is ridiculous. Actually, scratch that as well -- The whole hardware is ridiculous. I know I seemingly said otherwise in the previous paragraph but whenever I really start to think about it, I get upset at how big this console is. Even Kinect is heavy.
The load times are ridiculous. But they can always get that down in future patches. And I agree with others: There needs to be some kind of "loading" indicator rather than just sitting on a static green or black screen.
My final assessment of XB1 is "Ugh." It has a lot of potential to grow and become amazing, and I have high hopes for it. For now, Killer Instinct is my favorite "next-gen" launch title between Xbox One and PS4. I'm happy Microsoft took the chance at brining it back.