Installed in VirtualBox without much fuss. Chose Windows 8.1 (64 bit) as the type of virtual machine, changed it to 2 CPUs and enabled PAE/NX. I turned on 3D acceleration, but you can't install Guest Additions yet, so that's not going to do anything at this time. This means that it's using generic video settings in 4:3 or 5:4 resolutions. It's a good idea to turn off the animations (PC Settings > Ease of Access > Other options), since your CPU is going to be rendering them until Guest Additions is updated.
I'm not a fan of squarish, two-color interfaces. It effectively removes all the visual cues which tell you what things on the screen are actually supposed to be. I realize nothing is set in stone yet, I just hope they at least give us the option to switch to something else. It doesn't have to be all plasticy like Aero was, either. I suppose these are basically Windows 8 complaints, so I'll move on.
The Live tiles in the Start menu are neat and I can see them being useful. I like where they're going with the PC Settings window, especially for new users. I love how you can snap windows into four corners - that'll get plenty of use. Multiple desktops is also welcome. Getting a free 15GB OneDrive is nice, and the monthly pricing for more is fair ($2/100GB, $4/200GB, $10/1TB + Office 365). I had an issue where instead of saying "File Explorer" in the Start menu it was displaying some garbage, but I clicked it and it was able to fix itself once I pointed it to explorer.exe (it had to ask?). Horizontal scrolling in the Store app is terrible. That's all I can think of for now.