So, perhaps, if this point keeps coming up again and again and again, it is an indicator that it is a real pain point for the users?
I suppose I just don't understand from any of the discussion I've read so far what the theoretical drawbacks are going to be. So, the tag space expands, so what? It's just adding dimensionality to the content space, adding far more aspects to view the feeds through.
As you say, I want to follow the tags that are relevant to me to catch what I think is interesting - but if everyone tags every music post with #music, even if I follow #triphop, it is useless because nobody uses it. It's a chicken and egg problem in that case. Alternatively, if I am interested in #elections but not #politics, I am going to miss out on any posts that are relevant to both because the original poster decided to use the more broad category. Or what if there is a great #NASA #comedy music video?
In short, with the current system I see only limitations, but no advantages. As an engineer, building this type of system I feel that I would either remove the tag system altogether and try to work out some other approach, or remove the crippling limitations :)