If you were to install a leaked or archived version of build 9650, the immediate takeaway would be how much it resembles Windows 8.1 rather than the Windows 10 we know today. However, there are subtle under-the-hood changes and UI experiments that hint at the future.
The single most important change in build 9650 is hidden in plain sight: you can now resize Modern (Metro) apps like desktop windows. In Windows 8.1, Store apps ran full-screen only (or snapped to the side). In 9650, you can drag a corner of the Mail app to resize it arbitrarily. The title bar still uses the massive 8.1-style font, but there is a new close button (the traditional X) in the top-right corner. This was radical. This was the precursor to the Universal Windows Platform (UWP). windows 10 build 9650