Not OP but I also had a Windows Phone (8). It had a consistent design in every app and it just "felt" good to use. The inboxed apps were great (surprisingly!) and the phones themselves were quite cheap. Windows Phone was optimized for low RAM phones so it ran smooth there. I also miss the keyboard, I rarely mistyped which happens constantly with GBoard. Haven't found a replacement that doesn't suck yet...

It's hard to put into words because you have had to use it to understand!

I despise Windows 10 Mobile though - it killed the whole platform and made the OS unbearably slow. It was full of bugs and it was clear that it was the beginning of the end for Microsoft's mobile amibitions. I use Android now but it's a mess. Google can't decide on a consistent UI + UX and everything is all over the place.