Both sites are centered around the discussion of a link or topic. Reddit is mainly about submitting links, but the real social aspect is about the discussion around a link.
Branch is basically the same, but maybe even better. The latest complaint from reddit's users is about the misplaced focus on user, post, and comment karma; Branch does away with that, and the value of a branch is only measured by the amount of discussion in it. People can "branch off" of a certain piece of a branch to discuss about that.
It's certainly a long way until Branch has the type of community that reddit has, but it might get there.