First off I do I like the startup speed improvement of Windows 10 over Windows 8.1 (which was pretty fast itself) and I think Win 10 is generally a smooth operating system with what works. I also think the start menu in Win 10 is much improved over 8.1, though Classic Shell made 8.1 totally fine and does the same for Win 10. That said, I much prefer any version of Windows over Win 10 and I don't understand Microsofts one platform strategy at all in practice. I also did not like how aggressive Win 10 was pushed to Win 8/7 users with notifications that you could not remove from the tray without uninstalling/hiding updates.
Edited: This was pretty long and rambling so I made it smaller and to the point.
What I don't like about Windows 10:
1. The OS seems very partial to mobile concerns and touch screens with live tiles, apps (background apps), touch/ipad like switches, cortana etc. Aside from that fact that these features are not needed on a desktop OS, these specific mobile ideas have not done well to put it mildly on mobile devices. I'm afraid what has not done well in the mobile sector will affect the desktop OS long term, which has no need for background apps for example, for the worse. I don't like the one platform idea in practice and see it as placing emphasis on what has not done well and taking away or reducing what has done well. I also don't get the service idea of Windows 10 or how it is the last windows OS with no more improved windows versions that people could purchase ongoing.
2. I don't like the Settings system and not having control of updates and the need for a whole "privacy" section. Also, the OS to me looks very bland and flat and some programs on W10 seems slower than Win 8.1.
3. It's been buggy for me. Problems with email notifications, settings that were set off getting turned on somehow, a photo app that can't resize, game problems, privacy concerns. It's disheartening.
Personally I'd rather have a desktop/laptop keyboard/mouse OS that is all about that form and is optimized for the desktop. Not a touch screen compromise with features that have not even done well on touch screen devices.
I'm going to spend the money and go back to Win 8.1 and keep open to other options.