One super annoying thing I found is if you have any auto start programs other than Windows Certified programs like a VPN or something they tend to clash. My computer got worse and worse and eventually I had to downgrade. Also, I use 2 monitors and the start bar is on both of them. This is not something I like at all, as my start bar is on the left side and has been for 5 years. Any time I go back and forth from screens the start bar keeps popping up. The nail in the coffin is the complete invasion of privacy. There are so many ways in windows 10 that they are sending info to 3rd parties built into the OS. It has advertising built into it and seems like a useless upgrade. I feel as though Microsoft is benefiting much more from me upgrading than I would. They are probably making tons of money from these companies they are selling information too.
Case and point, they are giving it out for free in order to get as many people in their database as possible for their 3rd party leeches.
EDIT: I don't think I will upgrade.