Re: A very interesting detail - the root fs is per-user
The windows model doesn't have the concept of "root".
Users have attributes which make them root, rather than a fixed uid as Unix does.
In a way, it does. It is just hard to get to. It's called NT AUTHORITY\SYSTEM. That's your root equivalent. You can't directly access it but you can get to it by various means (PSEXEC -S being one method).
The Administrator account also has a fixed relative ID (-500, if I recall correctly).
However, in general usage you are correct. Administrator access is all via group membership (relative ID 512 IIRC for local admin access).