When I bought a WP8.1 phone I found it to be wonderfully innovative compared with Android and iOS, and immediately loved it. It made its two main competitors look like some clunky old fossils from the previous century.
The moderators of this forum have explained to me that WM10 has deliberately jettisoned most of the unique elements of WP8.1 in order to make the UI work much more like Android and iOS. Yes, that's right - it's a deliberate policy decision by Microsoft. Apparently by doing so it will make it much easier for developers to port Android and iOS apps to W10M.
The cost of that is that the UI now looks awfully like Android-with-live-tiles. Personally I hate it - if I'd wanted an Android phone I'd have bought one. W10M has given away almost all of what made WP8.1 distinctive and special, hoping it will now attract more apps.
I'll make a prediction: it won't. Microsoft have gambled their mobile phone crown jewels (the superb UI, graphics, animation and design language of WP8.1) on getting more apps, and I think they will lose. It's a massive shame.
Anyway, I rolled back to WP8.1 on my Lumia 1520 and it's gorgeous. In terms of the UI and design language, W10M is an ugly mess with almost no redeeming features.