If specificity were the only goal, then at some point you start to suffer from diminishing returns, as well as an increase in noise. The extent to which this would happen would probably be topic specific, however. That said, regarding specificity, I'm not sure where the average sweet spot would be.

But specificity isn't the only goal. Another goal is getting content to people that are most interested in it, without unecessarily watering down people's feeds. IMO the balance here is to get the post in the neighborhood in such a way that a typical following pattern would appropriately pick it up. That is, if I am interested in Space Exploration, the tags that I will most likely follow should be enough to pick up most Mars Curitosity posts, without having to also draw in a number of non-Space Exploration related posts. Community tags was an effort in this direction, but as lil mentioned, it frequently wasn't enough, and I also found myself often wishing that I could add just one more tag to appropriately label the post. Sometimes I had to choose between labeling and editorializing for a particularly odd post.

Another problem that I see with a large tag count, is gaming of tags. The less each individual tag is worth, the more people might be tempted to use the tags to optimize viewership. For example: #mars #space #upskirts, which wouldn't work too well on Hubski, but you get the point (maybe: #mars #space #poetry).