If there's one thing I don't like about Windows 10, it's the visual style. My windows look fresh out of Windows 3.1. You can modify the colors, but it changes everything. The visual style, at least for the windows themselves, is a step back, I feel. For example, when I change the appearance of my browser, no matter what color I change it, the _ [] X control options in the upper right hand corner remain the same color as the current Windows theme. So a black theme for Firefox gives me black, and then a poorly cut out block of some other color that clashes with the theme. Also, when you change the color, those control options remain black like text, so the darker the color, the more likely it is you won't be able to see them.
I'm hoping some solid visual styles come out for Windows 10. I'd love to keep the visual style of 7, personally, as it had the nicest contrast and colors, I think.