What are the benefits of the new and changed features?
New Control for UAC’s allowing user to configure the way that UAC work.
For Server 2008 r2 the built-in Administrator account in Windows Server 2008 R2 does not run in Admin Approval Mode The built-in Administrator account in Windows Server 2008 R2, which is the first account created on a server, does not run in Admin Approval Mode. All subsequently created administrator accounts in Windows Server 2008 R2 do run in Admin Approval Mode.
For Windows 7 the built-in Administrator account is disabled by default in Windows 7. If the built-in Administrator account is the only active local administrator account during an upgrade from Windows XP, Windows 7 leaves the account enabled and places the account in Admin Approval Mode. The built-in Administrator account, by default, cannot log on to the computer in Safe Mode