To evaluate how successful W10 will be, you can't view it with your own eyes but with the eyes of the average idiot user who has no idea how any of this stuff works (and doesn't want to know). That's where Apple succeeds. It's simple. Anyone can buy an Apple computer and even without reading the instructions sort of figure it out on their own. Apple's OS is like the dumb blond of OS's - good to look at and easy to use.
This is where W8 failed. Instead of the dumb blond it was the hot lab assistant. You wanted to fool around and she wanted to discuss astrophysics. It wasn't intuitive to the technologically challenged. Too many dead ends, too much non-obvious functionality. If your OS is confusing in the first 5 minutes of use, you have a problem. I was standing at the counter at BestBuy when a woman brought back her new W8 laptop. She said, "I need to return this," why ma'am?, "I can't make it do anything!". And there you have it in a nutshell.
If MS can make W10 idiot-proof while also providing the speed and power required by power users and enthusiasts, they could have a winner. Don't know if MS can do that. Hope with new leadership they can.
** The flexible user control and theming of the Start Menu is a step in the right direction, but they still have a way to go as far as functionality. C'mon Microsoft, you're dancing around the edges here - show us that you get it!