Is there an NT Account Operator equivalent in Exchange?
Yes, in a manner of speaking. To give a user, such as an assistant, permissions to create, modify, and delete Exchange mailboxes, you need to give that person what's called "View-only Admin" permissions.
To do that, start the Exchange Administrator program and select the Exchange Site. Click the properties icon on the toolbar (or select File > Properties) and go to the Permissions tab. Give the Windows NT account for that user "View-only Admin" permission. Click OK to get your way back out of the Site properties.
Next go to the Recipient container and add the NT account to the "Windows NT accounts with permissions." Make sure to give it "Admin" permissions here.
If you want to give this capability to multiple users (or if the user involved is going to change periodically) then I suggest you create an NT group called "EAccountOps" (or similar), assign users to that group, and assign permissions in Exchange to that group. That will be easier for you to administer.