I think,
1. “New” Windows - just because it’s a new version
2. “Cheap” Windows - cheap upgrade price than ever before. of course it’s until 31 Jan
3. Nice Metro interface design - design is good. but it’s not good for desktop usage IMO.
4. Good benchmark - of course it need more observation, but some guys saying it’s a slightly light and speedy than 7.
5. etc…
I think Windows 8 is not so good and matured OS, but MS sell it very cheaply and that went over big.