I'm a dual Mac / Windows user. Always had Mac laptops since 2004, always had Windows desktops since the late 90's. I'm a Windows 8.1 user and I'm a Mavericks user.
In my opinion Windows 10 shows a definite shift in Microsofts approach to their user base. They tried to do something different with the "Modern" UI. It didn't pay off.
It had some great features like live tiles but it just wasn't usable on a desktop or laptop without touch. I've been using Windows 8 since it launched and I've only ever tested Modern apps. And by that I mean I tried a few, I didn't like them. A few months later maybe I tried a new one, never used it more than a few minutes.
On my day to day system where I get stuff done, be that software development, image editing, gaming, server administration.. whatever it is, I'm never using a Modern application. And that is because they aren't designed for a Desktop system.
Full screen doesn't work when you're sitting in front of a 27" 2560x1440 display or in my case three 30" 2560x1600 displays. It looks silly, just like when you run iPhone applications meant for the iPhone 5 on a 6 Plus. It looks stupid. The applications have to be optimised for the screen size they are running on.
And now it looks like Microsoft gets that. I'm looking forward to Windows 10 for a lot of reasons.
A start menu that isn't just a "We know you wanted it so we brought it back" but is actually an improvement over the one Windows 7 had. Live tiles are a great idea just not when they are full screen.
Drop shadows on windows, whoever thought it was a good idea to remove those in Windows 8 got it wrong. It sometimes is very difficult to tell at a glance which window is in focus because there is no drop shadows. This was not an issue in Windows 7 or OS X as they both had noticeable drop shadows.
And I also like the new Alt-Tab interface. This is a feature I use all the time in OS X but on there it's really the Expose feature that is now a part of Mission Control. I really like that presentation style of application windows and I'm glad Windows 10 is adopting it.
I'm sure a lot of people are going to rag on Microsoft for copying Apple or giving up on their vision, going backwards instead of forwards etc but as an actual user of their products and every OS since Windows 95 and also a very happy Apple user I think this is great stuff. I'm really excited for Windows 10 and that is the complete opposite to how I felt when Modern UI was unveiled with Windows 8.
Just my thoughts