I am by no means a Windows fan (Linux only sinds 2003) but Windows Phone/mobile was (and is, I have one still) awesome. Snappy, no frills, yet feature complete. Pretty interoperable. Pretty. Pity it's over.
The snappiness of apps was also hindered by network I/O in the UI thread (I mean wtf Microsoft).
WP8 did some things better but it was too little, too late. Nokia at some point bitterly complained about Microsoft's inertia in getting important features implemented that Nokia needed in order to sell phones (besides the apps, but that is another whole can of worms). https://www.techspot.com/news/53405-microsofts-slow-developm...
And it is not that the WP8 release fixed everything that was wrong. Rather, it added to developer churn by switching to yet another API, it upset users because no WP7 phone could be upgraded to WP8 (except for the HTC HD2 which originally came with WM6.1), and it still didn't support any modern mobile web browsers.
Then there was this strange limitation to only 720p screens and dual-core CPUs until WP8 GDR3. Or that you could not disable automatic screen rotation until that update. VPN support and the notification center came only with WP8.1.