I believe that a really useful administrative facility that should be built into Microsoft Windows is the ability to login as a user by using an administrator's credentials.
System Administrators would often benefit from being able to log into a user's account to test the user's environment. Currently, this would either be done by either knowing the user's credentials or by resetting the password on the user's account to gain access. The former method works well for small business, but does have obvious security implications. The latter is a pain for the end-user when "strict" password procedures are in place governing their use of previous passwords and the like.
I envisage that an administrator faced with the CTRL-ALT-DEL dialogue should be able to press a button to extend the dialogue with an extra "Login As" textbox. They should then be able to provide their own username and password as well as the account name they are intending to login to. I would expect that this would require the administrator concerned to have been granted a "proxy right" and that its use would be written into the system's security event log.
This would then allow an administrator to use the account without any elevated privileges. It would also be useful if the administrator had the ability to login and opt out of group policies or to login to the account with their own administrative rights.