I found it interesting that the new UI ditched Metro with the exception of live tiles. It looks a lot closer to Google's Material Design, which I think is good. The best Metro has ever looked was in the old Zune desktop app. Everywhere else it looks a bit too spartan for my tastes.
That said, the rest of the demonstration just looked like a .1 increment rather than what I'd expect from a full version upgrade.
My biggest concern is with the carriers in the US. Specifically, Verizon (which I'm on). They've treated Windows Phones pretty poorly. They're currently not selling the Lumias or HTC models, and they've been painfully slow with the 8.1 update. 8X users still haven't received it through normal channels.
I love the Windows Phone platform, but if they can't maintain a variety of devices or quicken the updates with Verizon, then it's really all for not. I'm actually going to jump ship to Android specifically because of those reasons. I don't want to, but I don't live in an area that has good coverage by anyone but Verizon, and I'm tired of getting the short end of the stick from them because I prefer a less popular platform. I honestly hope that W10 phones take off, and that when I'm due for a new phone in 2017 or so the platform's ecosystem is a lot more robust and stable. I will definitely come back if there's a future in Windows Phone.