I just have a sec before I am supposed to watch the Walking Dead with my wife (It's awful and good). The short version is that a tag gets too many subscribers, and too many posters. Soon #space goes from discussions of volcanism on Titan, to countless Neil deGrasse Tyson love-ins. Everyone is soured by the quality of #space and declares Hubski to be full of mouth-breathers. A few survivors create #realspace, and it begins again.
All right, thanks for that. The short version of my answer is the same as the long version of my answer: If you clutter up my #space feed with Neil deGrasse Tyson love-ins I can ignore you. Subreddit crawl is a problem in that subreddits are one-dimensional. By allowing your users to subscribe to both tags and other users, you create a two-dimensional subscriber space. Granted, if a person posts three lame things and one awesome thing I might miss the awesome thing because I'm ignoring them - but if I see one awesome thing in a miasma of crap I can follow the person who posted the awesome thing without having to worry about the miasma.
Anyways, I think hubski naturally promotes content good for discussion. Since tags can be used by users to mark bad content (ex. #spam or #repost or #loweffort), I think it would be pretty easy for the community to regulate itself, provided that it doesn't get flooded with people who like bad content, in which case almost everything is useless to combat it.