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.