They're engineering the compatibility of Windows 7 to be exactly that of Vista (FWIW) - system requirements and drivers should be the same. Microsoft is also hoping to make a dent in the netbook market with Win7, so the OS itself should be light enough to run on lesser hardware (probably without Aero though). The way I see it is Win7:Vista::Win98:Win95... a few new features, but visually and under the hood it's pretty much the same.
I downloaded the Win7 beta and installed it to a VM earlier this month, and it's been pretty fun to play around in.
- Driver compatibility seems to be OK. Since I installed to a virtual machine it's kind of hard to tell, but the VMware Tools virtual hardware drivers haven't yet been updated for Win7 and they worked. However, since VMware doesn't support DirectX I couldn't use Aero.
- The Sidebar is gone, but individual Gadgets remain.
- The new "SuperBar", which replaces the taskbar and Quick Launch, is pretty cool - it merges the best functional elements of the taskbar, Quick Launch, and the OS X Dock, and the final result is better than all three individually (IMO).
- User Access Control has four levels of configurable nagging, the highest of which is equivalent to Vista's nagging, the lowest of which turns UAC off completely. Even the second-lowest setting ("only warn when a program tries to change system files") never gave me a pop-up warning.
- As best I could tell, the "Classic Windows" (Win2k-style) theme and classic Start menu options are gone; I never found an option for these. They may have just been an omission from the beta. I've read that MS has locked out certain features of the beta to MS employees only.
- Nothing ever crashed on me.
So far, I would say Windows 7 has a lot of nice things going for it.