IE11 seems to be slightly different on Windows 7 to Windows 8(.1). I guess this is because some features are tied to the OS.
E.g. the IE11 on Windows 7 developer preview blog post briefly mentions SPDY:
On Windows 8.1, IE11 also supports the SPDY network protocol
And DRM video:
On Windows 8.1, IE11 supports the latest media streaming standards, Media Source Extensions (MSE) and Encrypted Media Extensions (EME).
Are there any other differences that web developers need to be aware of (especially CSS and JS APIs)? Is it worth downloading 2 separate VMs for testing IE11?