That was hands down the best UI experience I've had to date with a phone. Nothing has come close.
The live tiles were very useful and gave you basic information at a glimpse with ANY app you chose to pin on the start menu, and a system-wide dark mode was available years before Android and iOS had it. The Windows phone had unified messaging, and Cortana was (maybe still is if it still works on the old phones) the best voice assistant. You could do ANYTHING with it, including singing random songs which was hilarious, and still be natural sounding. She could pick out a song on the radio, and link me to the song on Zune where you were able to download the album from the store. It was all integrated, and it was years before iOS and Android adapted that; heck, I remember when SoundHound was a thing.
It was the closest we non-iOS users came to PC/phone integration. I had installed a podcast app on my phone and PC, and used Cortana to remind me when a new episode came out on whatever device I was actively using at the time.
One more thing, the cameras on any windows phones were amazing, you were able to do so much within the stock camera app. I could zoom in for days on my photos and still have clarity too; and it even recorded audio in Dolby 5.1 surround sound, which made my videos sound beautiful on a home theatre system. It was cool.
I think the main problems were the lack of apps (that's what did it for me) and it was too ahead of its time. It scared a lot of people to have that level of integration which is ironic...
Honestly, Windows phones help shape the mobile OS's we have today to be what they are.