Hmm. I'm still not sure a personal tag would reach a large enough group to be feasible. Initially I saw it as a way to archive your own posts neatly. Which I would do with my own content, but wouldn't with things I merely posted for everyone's enjoyment. I'll think about it.
I think both use cases exist. For example, I am pushing for the ability to add a personal tag to other people's posts. Why? Because I would like to be able to create archives such as #bestof2013.thenewgreen and go through the years posts and pick out my favorites. I would love to archive hubski's best content this way. I'm not sure people would "follow" that tag, but I would definitely share that link w friends etc outside of Hubski.
I agree, you could go in to someone's profile and see "personal tags"