Ste as far as we know right now it should use the covented WinFS file system. The one that was supposed to be in Vista but was dropped. It will be a file system that can be index easily by Windows and used to search instantly.
The rumors right now are few. There are only a couple about increased security, closed kernel where as there will be no root access to the kernel like OS X has at this time.
As for inmprovements over Vista who knows at this time. As it isnt not even in Beta stages.
Same goes for the versions. It could be 5 like Vista or less like XP with Home or Pro. Vista had 5. Basic, Home Premium, Business, Ultimate and Enterprise.
How it compares to Server 08 again is unknown at this time. Will have to wait till the further stages to get more info.