My thoughts, after briefly playing with the preview, is that Windows 10 will be the Windows 7 replacement that Windows 8 never was.
That's not to say that it's anything special -- the changes are more evolutionary than revolutionary -- it's just generally more usable than 8, especially for desktop and business users.
To be fair though, what else is there left to do to improve a modern OS? Most changes now are cosmetic, and even those are more about fashion than function. We've already gone full circle in that respect, from the plain, flat appearance of earlier Windows OSes, through the graphically 3D "Wow!" of Windows Vista and the more subdued Windows 7, back to the flat, retro-look of Windows 8 and 10.
I'm not sure there are even enough changes under the skin now to justify a whole new Windows release from 7 to 8, let alone from 8 to 10. I think Windows 8 should really have been Windows 7.1 (broken edition), Windows 8.1 should be Windows 7.2 (new, improved flavour edition) and Windows 10 should be Windows 7.3 (fixed edition). But of course, that wouldn't have been good for business; old rope is much easier to sell disguised as new rope.