I'll be upgrading. I'm one of the few weirdos who doesn't actually mind Windows 8.1. Sure Metro sucks on a desktop, but I rarely need to see it and it's not proved to be such an inconvenience that I have to install Classic Start Menu or similar. I still think Windows 7 to Windows 8.1 is actually a worthwhile upgrade, even if it's only for the lower system requirements.
Hopefully Windows 10 will continue the trend of reducing resource & power consumption (or at least keeping it very low) - I'm particularly looking forward to getting a free upgrade to Windows 10 on my new Linx 7 tablet
(assuming that this will indeed be possible!)
More importantly, Microsoft so far haven't specified whether DirectX 12 will run on Win 7/8/8.1 or whether it'll be Win10 exclusive. If DX12 is a Win10 exclusive then that alone will be reason enough for me to upgrade - DirectX 11.1 support was what finally pushed me into upgrading to Win8. DX12 looks pretty damn swish so far, especially considering that it won't require a GPU upgrade for many people.
I'm also quite intrigued by HoloLens. I was skeptical as to whether it would actually be possible to display a stable rendered image over real-world objects (i.e., turn your head away from the table but the "holo" image stays in the same place), but recently I've seen the augmented reality effects that are possible with a modern smartphone (Sony Xperia). Kinect and the acquisition of Minecraft (via Mojang) is suddenly starting to make a lot of sense - an augmented reality version of Minecraft would have me selling body parts to buy a HoloLens ('cos they aren't gonna be cheap!), and I suspect that I'm not the only one who shares this opinion.