Despite some of the stick I got (completely wrongly) in the Windows 10 thread about not liking change, etc. I was one of the first to try Windows 10 - with it installed on multiple PCs and 2x VMs and suffice to say I've put everything I possibly can back on Windows 7 or 8 the only devices I have on 10 now except a couple of Atom Z8750 tablets which only have Windows 10 driver support
-Failed updates, incredibly slow updates and/or lots of oddities after updates
-Start menu or explorer not infrequently become unresponsive often only resolved by reinstalling
-OS deciding to do things for itself despite lacking real life contextual awareness - whether that is background processes causing stutter or restarting of its own accord, etc.
-Stupid implementation of updates - I mean who the **** has such a shuttered life they think people have a constant 9-5 seven days a week routine i.e. active hours
-Pitiful lack of attention to how the OS works on portable devices meaning you can get stuck on "don't turn off your PC" while your battery is draining, etc.
-While subjective personally I think it looks ugly as **** and the monotony and inconsistency of the visual elements and the clutter of stuff like controls in explorer fatiguing when used for long hours compared to most alternative OSes - even most flavours of Linux look less amateur and less like they UI was thrown together by random people on their weekends, etc. these days.
Most of this has been repeatedly fed back on the hub on insider builds, etc. but ignored - MS only listens to what they want to hear - shame because its a potentially good OS let down by a lot of poor execution and general unreliability.