I think that having thumbnails promotes image browsing/posting and just skimming through posts. It adds to the instant reward complex that many that submit images on Reddit seek. The lack of thumbnails here is one of my favorite things about Hubski, it promotes not only reading the article but also the comments.
I think that prior to the multi-reddit addition Hubski clearly had more freedom, but now I would have to agree that it is a pretty even experience in that regard. However, I do very much enjoy being able to follow domains here.
To me, followers and following people is huge. Who you follow is just as important as what tags or domains you follow. I've found so many interesting submissions that weren't part of the tags I follow by virtue of following a diverse set of people here.