As a daily windows user I'd say the main problem is hidpi support. It can't do mixed dpi environments properly, which is the typical experience when you use a (hidpi) laptop combined with an external monitor. Microsoft blames this on legacy software, but even office doesn't work properly.
A secondary issue are forced updates breaking things, which i've had on two of my 4 windows laptops so far. Finally, the mess of legacy and new settings screens is annoying, but once setup it doesn't matter much, so it only affects the first-run experience.
Aside from those i think w10 is nice. I wouldn't mind replacing my mini with a windows machine, and if apple doesn't update it soon i probably will.
The short version is that the Windows ecosystem is plagued by legacy software (even Windows itself).