The whole "flat" Windows 10 theme is really bugging me. When you have multiple windows open and overlapping it can be really hard to tell where one window starts and the others begin. That makes resizing or dragging windows by the title bar tricky. I wish there was some way to add more separation between them.
I've also got an issue with the way some of the control panels are opening. There is plenty of screen space, but some of them don't open wide enough to display all the content, leaving a horizontal scroll bar.