I guess like Paul says if you don't like nicer things and want to be on a faster more secure O/S.
a) Although there are some nice (in terms of looks) things on Win10 (and I am ONLY thinking about the -buggy- taskbar, the action center and the explorer window here...), there are many other things that are terrible (the rest of the stuff, I would say...). And I really don't like the default settings for the taskbar to combine the taskbar buttons. But anyway, they have made that customizable so I won't nag about it much... But, please, for heaven's sake, I don't want to click 2 times to restore a window. I want it restored with one (or less) clicks. Even WinXP could do that. And you still share the same shell bugs with WinXP. When will you fix that shell of yours? Window restoration and minimizing has still bugs in it. And now there are new bugs related to modal windows mistakenly placed behind their parent window (you can understand what this causes, don't you?) !!!
b) Win10 is not faster. In fact, with the exception of Win7, every new MS OS is getting slower than the previous version (especially if you are not using a SSD). At least that is my experience having used all of them. If you have any doubts, install WinXP on a VM (which should be 5 times slower that running on bare metal) and see how long it takes to boot, to start launching applications, how often it swaps memory from the paging file, etc, etc.
c) When it comes to security on Win10, I don't feel secure at all. I know that once I exhaust my RAM it will stop responding and it will either freeze or crash and reboot (I don't even get to see the warning for low memory displayed - that's how fast it crashes). Then, I am afraid to reboot my machine, because I don't know if my taskbar will be functional after the boot. On top of that, it seems that the whole OS was built for the purpose of collecting user data (which violates privacy big time). So, with all these fears in my mind, what on earth do I have to fear from a hacker? Not to make my PC a DDoS attack endpoint? I don't freaking care! It's not illegal to get hacked! Not to steal my credit card number? I don't use one! Not to steal my email passwords? What the hack is he gonna do with those? Read my spam? Blackmail me for the evil spam emails that I get? Certainly he wont make a profile out of my data. He will just get what he likes (if he finds anything), he will leave, and totally forget about me afterwards. And even if he is j*rk enough to write ransom-ware, he is not gonna get any money from me, I have my files backed-up and he won't manage to encrypt more than 1% of my files before I find out about it. And anyway, Win10 is not so improved on security to make someone upgrade to it. You can still hijack apps that listen on the Internet and there is still code from earlier versions of Windows in there (much of it, judging from the same bugs that I see) that can potentially be exploited.
Nevertheless, I have upgraded to Win10 and I use them every day. It's been less than a month now and I have already lost count of the number of low-memory crashes and the times I had to reboot to get the taskbar at least semi-functional. And not to mention that I had to run a 3rd party driver-updater tool (the stupid computer management said all my drivers were up-to-date... :|) to get my screen resolution setup correctly, so that not everything looked like sh*t. I wonder what simple users did when they faced such situations (that might explain the rise of sucky crApple to the top, despite the fact that both their designers and their developers are untalented and uninspired)...
I hope MS fixes those, so that I can say something good about it the next time...