I used Windows 10 as an "insider"(alpha tester) from build 9926 all the way up to, and just past the RTM date.
1. Window borders are too thin. Makes it difficult to discern windows when they're slightly overlapped.
2. Title bars are the same color between active/inactive windows.
3. Can't control/ very limited control of Windows updates. Meaning, you can set a time for it to restart, and "Active Hours"(but you are limited to 10 hours,) but if you have the "Home version, and you do video rendering that has been known to take DAYS, you are screwed. (heard it happens to the PRO version too)
4. Media Center: You can't have it. PERIOD! While I use rarely it, I have several friends that stream video through their homes with it. SHOW STOPPER, literally!
5. Your peripherals may no longer work, even with "updated" drivers. I was never able to successfully print with Win10 to my 3 year old Brother laser printer
6. Menus aren't consistent across applications, and that confounded POS ribbon in Windows Explorer(or what ever Microsoft is calling it now). Didn't like the ribbon when introduced in Office, still don't like it!
7. Limited color pallet. Don't even think about changing window background colors, you can't. Your only option is stark white background with dark gray text.
8. System fonts: You can change the size, but what ever font Microsoft has decided what THEY want you to use, you're stuck with it unless you change it in the registry.(but you have to find it there first)
A year after release, I feel that Windows 10 is only in the BETA stages, and won't be ready for "prime-time" for a couple more years.
People flocked to Windows in large part because of the manufacturer partnerships with Microsoft, but also too, because Microsoft allowed us to "truly personalize" our PC's, unlike Apple.
To say the least, when I go to replace my laptop, it won't have Windows 10(or at least not for long, and it won't ever be registered that way either). The same goes for when I build a new desktop machine. If I can't get Windows7, I'll find a version of Linux that I can live with, as I don't want a phone UI on my PC's.