Hubski has different mechanics than those sites. On Hubski, your main feed is made up of posts from people you follow, and the posts of people they have chosen to share. The basic philosophy is that quality conversation and content comes from certain people, -not certain topics. I think you're probably familiar with the problem of subreddits on a range of topics devolving into crap. What if you only followed the best posters on say, r/technology, instead of having to see everything that the crowd up votes? That is more what what Hubski is about.
If you like topics, you can follow people who post on that topic often, and as a nice bonus, you also get to see posts on other topics by people of quality you follow. This is an elegant solution to the problem of new content discovery and not getting locked in to just the topics you've selected.
As others have mentioned, the community is smaller, but that won't last forever, as Hubski is growing. The beautiful thing is that no matter how big Hubksi grows, you can keep it as small as you want since you decide how many contributors make up your feed. It could get huge and you might never know it. I think it will scale really well, but that remains to be seen of course. Also, you can block trolls and antagonists from commenting on your posts if you wish.
Also, the 'up vote' system is not designed to encourage karma-whoring. It stops giving you vote totals after 8 votes, and there are not downvotes. This speaks volumes about the character of the site.
There are many other differences that I won't go into. In short, you should use Hubski because you've been curious enough to come here and ask why you should. And you should keep using it if you dig it :)
Lastly, if there are any features of Hubksi that you like, don't get too attached. Hubksi is young and developing pretty rapidly. Features come and go pretty often as the site is not shy about trying new things out. This is a good thing.