It varies from machine to machine but I've run into this with causes being an antivirus suite, some USB HDD, printer, other thing plugged in to registry cleaners. As I can't check those for you, you have to think about what's not stock and keep thinking that the upgrade expects a plain OS and lots of free space.
There's another issue I can't tell if it applies. Some early W10 adopters have the old license/version of the free beta days. That license will die and you can't upgrade those. That can be very upsetting to those that thought they were getting it for free.
I am one of pioneer users of windows 10. Now I was suggested to upgrade to PRO version. Trying to do so the process was very long and was stuck at 47% of it...after few hours switched off my PC. Lucky after 5 min computer managed to restore my last version....I didn't give up and after few hours tried again and reached same point of 47% done....can some one comment on that ?