Reddit's choice to use sub-communities, rather than tags is what differentiates it from all the other link aggregators out there. Tags are public property but communities are controlled by the mods and the members. Each community has the power to make and enforce its own rules (like no memes). When one community starts being diluted by an influx of users, the core users migrate to another community. With tags, it's not easy to convert someone using #physics into using #real-physics instead. Reddit will always have these unique communities these will be what keep people on Reddit

Hubski is closer to Tumblr (or Twitter or the stream component of Google+). It's mainly about following people or browsing hashtags. Hashtags are good to follow at the start - but eventually they'll get diluted. What is critical in this model is giving users a way to find people to follow. Twitter has this covered because there are a ton of celebrities on it and most people will have a few friends on it too. That said, Twitter offers friend lists that allow you to follow a bunch of people at once. Tumblr is also heavily based on following your friends, but they have a spotlight (http://www.tumblr.com/spotlight/travel) to highlight which users in a category may be worth following