What's so great about Hubski? Well, I for one, actually feel like a member of the community. I think this is because, as thenewgreen said, users tend to interact with each other much more often. It's a site driven by user interest and it functions by following those areas of interest and/or particular users. By being able to follow people with interests similar to mine, I am then able to share things with particular people and have real conversations about topics, similar to the way that I would when interacting with people in the real world.
I really like reddit, but as far as discussion goes, I like Hubski by a few miles. A lot of active users post original content as well. People are willing to put themselves out there and to explore topics in a way that I rarely see on reddit. I feel like I could get along with a lot of users on this site in real life, which is not a feeling I often get from reddit. To me, that's a big deal, because I'm pretty "whatever" about a lot of people I meet.