I'm all for moving forward with design and user interaction and I'll reserves my opinion on W10 until it's released. BUT so far the feeling I'm getting from users of W8.1, MS & the preview builds is that a lot of the cool stuff, the stuff I felt made me more productive, from 8.1 is going away. If it does then I'll be a bit annoyed. I was already bemused by people saying W8.1 was a failure, but whatever, don't use it. Now though those sentiments seem to be screwing up my experience too. I had a good thing but because people hate change I'm losing it.
I find Windows 8.1 works really well across both tablet AND desktop, I'm always marveling at the way I can easily do things like sharing between apps & to my Xbox, searching apps, files & the web, transitioning between desktop & tablet with my SP3. I even like using the Modern apps in a desktop sense because of the adjustable snapping and the sharing between apps. I feel more productive.
Anyway, like I said, I'll wait a few builds and see how things progress before I install on my Surface and then hold back my final opinion until it's complete and I've had some time to get used to it.