The OS was smooth and worked well, and the design philosophy across basically every app was extremely coherent. Everything worked well, and everything felt like it fit together. Really its only problem was the lack of apps due to companies intentionally not supporting Windows Phone (Microsoft had a YouTube client that Google made them kill off, and Google never bothered to release a replacement IIRC)

It was as though MS actually built something amazing and Nokia was back to its roots of building solid, well designed phones.

UI was very forward thinking in the right ways. Buttery smooth, etc.

Of course it flopped because a bit late (arguable) so it lacked apps.

No idea why this couldn’t have been brought to android at least as a launcher or provide some kind of support to run Android apps. Just MS being MS —ordinary users/consumers aren’t their primary target in general.