As the OP, I never accepted Ed’s “solution.” He forced the acceptance as a HubSpot employee. I have since unaccepted the solution.
I consider any custom property-based solution to be a parital one, at best, since it must be created, managed, and tracked separately for contacts, deals, companies, etc.
A true tagging system would be global across the platform, allowing the tag applied to a contact to be visible while looking for that contact’s deals, and vice versa.