Don't upgrade if you have an intel HD Graphics chip: I have a HTPC that ran Windows 7 just fine, but Windows 10 could not play any video because they dropped support for the Intel HD Graphics Chip. While the OS does run, no video playback software would work. I even installed the drivers for Windows 7/8, but it still didn't work. I had to revert to 7.
Camera drivers: One of the two Windows 10 computers in my home can't talk to cameras that use PTP. (That's the file transfer protocol most cameras use. It's being replaced by MTP now, but a decade+ of PTP cameras still exist.)
3D printers, serial devices: The driver signing issues are going to be a big deal for people using 3D printers or any other USB-to-serial device.
Unrelated: Edge sucks. It's a tablet browser that is nearly useless on the desktop. Simple things like bookmarks and history are difficult to access without being able to "swipe" from the side.