In decreasing order of importance:
1) Mandatory upgrades / patches. Almost everything else in Win10 I can deal with, but this real burns me. I've been screwed over by Microsoft patches too many times in the past (not least being KB3035583 "Win10 Upgrade Notification tool") to ever accept this.
2) Privacy issues. No, Windows10 isn't as bad as was initially reported, but its defaults are still terrible, and even with it buttoned down it still leaks like a sieve... possibly worse, since I have only Microsoft's word that they aren't keylogging everything. And now you can't turn Cortona off anymore, so that data-torrent is opened right up again...
3) It has an ugly interface, where they've stupidly moved controls about for no real purpose other than to say they changed it, obfuscated commonly used controls, and removed the less-commonly used options so they can only be toggled via the registry (for now... they're recent policy change on policies indicates that even these options may go away soon). Sure, there are third-party solutions, but they don't entirely solve the problem.
4) It's very pushy with regards to using other Microsoft services, to the point where it cripples itself if you don't fully buy into the Microsoft ecosystem (try using the app store or onedrive if you don't use a microsoft log-in as your main credentials). And then there's that whole worry about the direction Microsoft is going with regards to service-based pricing...
5) Windows10 brings very little new to the table that I find of interest. It apparently boots faster, but my computer is up and running in 30 seconds already, and how often do I really reboot anyway? DirectX12 might be more exciting if there were any significant games using it. Better interactivity with an XBoxOne might matter if I owned an XBone, which I don't (and even then I doubt I'd care).
In the end, there's very little Windows10 has to offer me that earlier versions of the OS don't do as well, or better. Even if Windows10 was a compelling upgrade, I'd reconsider an OS upgrade if my current set-up is working well enough (as it is). But right now Win10 is just a little bit too ugly, too creepy and too pushy for me to want to have anything to do with it. Maybe in a few years it might be worth taking a look at but right now it seems far more bother than reward.