The UI seemed really good, I was really tempted.
But I was ultimately thrown off by App support in general.
Wasn't there a thing, that Apps often weren't equal to their Android and iOS counterparts?
Google actively blocked a ton of their stuff from being accessed from windows phones. They even deprecated some communication protocols in gmail to ensure some features of Microsoft's mail app would not work with Gmail (and then Microsoft found a way to make it work with an alternative protocol and Google went ahead and dropped that too).
YouTube could only be used in internet Explorer. Google refused to let Microsoft make a client for it or do one themselves. Some folks created third party clients for it but Google was quick to block them too.
Then Pokémon Go came out for iOS and Android and it was the nail on the coffin.
Edit: I forgot to mention Instagram, it was a big part of it too. I think this was before Facebook acquired them, because Facebook (and Twitter) were VERY well integrated into Microsoft apps on top of having their own apps available.
In general windows phone was the easiest platform to make apps for (I made a few), but there was a lot of sabotage from those big names. I was very conflicted because in one hand, Microsoft was tasting a bit of their own venom - as they had done the same sort of stuff so many times before, but in the other hand windows phone really felt like the best mobile OS and I wanted it to stay relevant.
Holy shit Pgo really was. I traded my LG lancet for an HTC One because pgo wasn't coming to windows.
And then it never fucking worked because Niantic "didn't anticipate the server load" of how many people wanted to play fuckin pokemon in the real world. That first year was a fucking shitshow. And I wouldn't trade the memories of walking around like a buffoon looking for a ghastly for anything.
I fucking loved when Pokémon go came it, it was so great, but a couple of months after it came out, I was going to the beach, and i was so excited. I figured, going to the beach, I would be swimming in water type Pokémon, then when I got there, I was like, oh, it's the same zubats at home. Never really played it after.
The HTC ONe was a gorgeous little phone. I miss it. One of my all-time faves and rock-solid build and one of the first to include an IR blaster - as the power button!