For me it was Windows 10 forcing itself onto user machines for months after launch, even permanently destroying user data to do so.¹ Oh, and the 260 character NTFS path limit, which bit me in surprising ways (MS published a reg hack to get around this on Windows 10², and I used rsync on MSYS2 to recover inaccessible data on Windows 7). I had been optimistic about W10 until then.

¹https://www.theverge.com/2016/6/27/12046738/microsoft-pays-1...

²https://docs.microsoft.com/en-us/windows/win32/fileio/maximu...