User Profile ModificationUsers are not nodes (unfortunately) and so we cannot apply taxonomy to them by default. However, we can apply taxonomy to users by hooking in to the user profile and inserting a taxonomy selector and then storing the results in a table that would resemble term_node.
User Node Access by Tag ConfigurationThe site admin would select a single taxonomy that would be used for user access control to content, lets call that vocabulary "employees". He populates this vocabulary with whatever he wants. For our example, let's add two terms called "manager" and "worker". The admin configures the employees vocab to apply to the node types he wants to restrict access to. He would probably also configure some module specific settings that would determine what to do with untagged nodes and untagged users (ie grant/deny). The admin now can tag users as manager or worker or nothing at all. Authors can tag nodes using the employees vocab as manager or worker or nothing at all.
ACL ImplementationUser Node Access by Tag implements its own access control based on the "employees" vocabulary. Access is granted where a user tag matches a node tag.
Of course all the complexities of ACL are not addressed here, but this is the basic idea. It seems simple and sound to me. I'm curious as to why nothing like it has been done yet.