Anonymous Ex-Microsoft Employee on Windows Internals
i.imgur.com
i.imgur.com
I especially loved the part where he explains that Metro was/is designed in PowerPoint. I did not expect that, but it makes perfectly sense now.
TBH the lack of testers shows: I used Windows 8.1 since it came out and apart from minor issues at the start (Start 8 is a godsend!), I never once had a BSOD... Win 10 gave me 3 in 3 months so I removed it.
UX testers on the other hand... not so sure.