What I hate about Windows 10 compared to Windows 7 is the flat theme. Personal taste aside, there is an actual problem in that for instance on a 40" 4K monitor where you have many non-maximized windows open at the same time, everything just blends together and you can't easily tell where one window title/border ends and another starts.
Whoever designed the flat theme had no clue about UI design and what important role depth cues play in recognizing objects, throwing out 20+ years of UI refinement out the window. It has zero to do with skeuomorphism, and everything to do with using the powerful ability of our brain to identify objects based on depth cues.
Note I'm talking about non-Sonar related work. Fortunately Sonar still has all of its depth cues in place (minus the title bar and border), so it is still a pleasure to use. Please never consider flat as the way to go in Sonar.
Other than that, Windows 10 has been working well for me.