The file Explorer is sometimes super slow (especially when dealing with media files, which it insists on analyzing first before displaying their folder), hangs on some FLAC files and frequently crashes.
It somehow can't handle pretty standard media files (again some standard FLAC that work everywhere else, variants of H264 or H265, etc.)
The networking works when it wants to - I still struggle to swap files between two Windows 10 systems on my home networks (one always can't see the other machine, and when I move files it's slow as molasses despite having blazing fast Wifi).
Work: Win10 fails to manage windows - doesn't remember where they go, can't reopen a set on startup. Don't allow pinning above other windows anymore. Opening a Word doc maximises the other 4 Office windows I have open and makes me spend time shuffling Windows around. So frustrating. There are 3rd party fixes for these things but work doesn't allow that, be nice if they just weren't broken. Win10 as a window manager seems very limited.
Home: For me the sound settings are weird there's no simple way to choose two outputs (headphones and speakers). I have to use a third-party app to make the volume range over 0-100 rather than 0-1 (1 is too loud, 0 is off). Adjusting mic settings is stupidly complicated, why isn't there a slider in with the output slider in the system tray (as I'm used to in KDE, and I swear they used to have in MS Windows?). Some apps it won't let you start as admin from the Start menu, but that works fine from Explorer, urgh.
Meanwhile it finds my portable Postman install no problem, even before I'd ever run it. I don't get it.
While, not on Windows alone, and I doubt this will change, I also think that close and maximize buttons on the corner of windows are too close that I had accidentally closed web browser windows many times. Of course we can just use shortcut keys, but I don't think that fixes root of the problem, and I am also now immunized on how window border buttons look that it might look aesthetically unpleasing if we depart from how it is now.
Many other software also suffer from this design quirks where buttons with very opposing outcomes are somewhat too close to each other, that you need to slow down and be careful.