I've been playing with Windows 10 developer preview in a VM and have upgraded a couple laptops and an old test machine from Windows 8.1 and Windows 7. All those upgrades went well.
So I was slightly surprised when upgrading my primary Windows 7 VM was met with a warning message that claimed it was not compatible with Windows 10. What? I was running a Windows 10 Preview VM on this same host a while back. The messages was "CompareEchange128", I'm sure my Sandy Bridge Core i7 would have that. It turns out that VirtualBox was configured for a Windows 7 64-bit which apparently does not support that instruction. Switching the config to Windows 10 64-bit solved the problem. The machine is now upgrading.