Well, with little help of @yeniaul I managed to get Windows Whistler to work on a VM, here's what changed, obviously not everything is here as I just started yesterday but I hope it's enough for now.
I'll be comparing a Beta 2 bulid to XP final SP3 (Both Professional)
Chapter 1 : Installation.
Difference 1:Different logo and loading bars.
Difference 2:Different boot screen.
Difference 3:Can't quick-format.
Difference 4:Critical error after the post-formatting reboot, the only way around it is to upgrade from Win2000 with it's system clock set to a specific date range. (1st January 2001 is confirmed to work)
Difference 5:Completely different Installation screens (Beta 1 only)
Chapter 2 : Introduction.
Difference 1:Different (logoless) AVI.
Difference 2:No introduction mp3.
Difference 3:A wizard instead of the help circle as your helper.
Difference 4:Different buttons.
Difference 5:A "mouse usage tutorial" if you wait long enough on the first screen.
Difference 6:System does not ask you to create user accounts, it does make one automatically with the name you registered the copy with.
Difference 7:System doesn't ask you if you want to enable updates.
Chapter 3 : Post-Introduction.
Difference 1:The system opens My Computer instead of the Start Menu.
Difference 2:A "Comment?" Button to the left of the buttons on all windows.
Difference 3: Different Desktop background.
Difference 4: Different Recycle Bin icon.
Difference 5: "My Documents", "My Computer", and "My Network Places" icons on the desktop active by default.
Difference 6:New shortcuts.
Difference 7:Text has borders around it enabled by default.
Difference 8:The taskbar has different design.
Difference 9:The taskbar is not locked by default.
Difference 10: Only one Luna theme color.
But there's more...