I think that what might insulate hubski a little bit from the proliferation of trashy content is the lack of karma. The culture, it seems, is more about sharing than narcissism, and the structure of the site reflects that. The promotion system is fuzzier and more obfuscated than reddit's, and that makes it seem more genuine.
I do think that moderation of hashtags over time could become an issue. With the ability to only attach one tag to a post, the really big tags like music, science, technology, news, politics, etc. will eventually get bloated. A potential solution is to strictly limit certain hashtags, or possibly automate the worst offenders. For example, if image memes were to take off here, they could only be posted in the #memes tag or risk deletion - and the tag would be autosuggested when the site parses the quickmeme url. That could be used to keep certain hashtags more 'true', without the need for multiple tags with varying degrees of purity (/r/gaming, /r/games, and /r/truegaming).