Why? Aside from the widely publicised problems, what actual positive things does 10 offer that previous versions didn't?
Cortana, like all the other personal assistant gadgets of recent years, seems very clever at first sight. However, I've seen little evidence so far suggesting that real users want this sort of tool or find these tools work well for them.
Edge seems to be unfinished and to have negligible adoption rates so far. This might change in time, but for now it seems to lack both the stability and reliability of IE and the flexibility and new features of Chrome or Firefox. It's not clear yet what, if anything, it will offer beyond these existing browsers to encourage users to switch.
DX12 is a gaming platform that so far has little support from either hardware or games. Again, this might change in time, but historically new versions of DX that were locked to new versions of Windows haven't been the driver for adoption that Microsoft might have hoped and in practice games have continued to support older versions of DirectX as well.
There are a few UI changes in Windows 10, but the positive comments about several of them seem closer to "this isn't as bad as Win 8" than "hey, this is actually useful". Other UI changes, such as splitting up configuration settings into lots of different places, are getting quite negative comments so far. So again, overall I don't see the UI being an advantage over other contemporary operating systems that might encourage people to switch.
So really, what is the killer feature of Windows 10 that would make a normal but well-informed user decide to install it on, say, an existing Windows 7 machine?