I agree with all of that. There is little doubt that this will alter people's tagging strategy. It will also significantly alter people's following strategies. Rather than follow #music, and filter users in their entirety based on what you don't like, you may look in #music to find the personal music tags that you want to follow, or filter out only personal music tags you don't, and not the users entirely.
Oh don't get me wrong I'm excited about the potential to further customize who and what I follow (as thenewgreen also brought up), I think my problem right now is more readability related with the way tags are displayed when every post has a personal tag. That's probably just me though.
It was one of the things that we struggled with most, and we tried a number of different options and formats. However, we always kept coming back to this basic presentation. IMO after using it a while, it starts to look natural as your expectation of the functionality adjusts.