I think that since hubski is so focused on discussion it those who like memes will not be as attracted to this site.
I also feel that our lack of thumbnails will aid us to this end a LOT. I think Reddit could be a totally different site if it didn't enable people to upvote memes without clicking a link into it, or without extensions like RES that make it even easier. Hubski's format doesn't really allow a path of least resistance in this manner, aside from the preview icon (which is also another click to view content). Someone who spends a lot of time thinking about UX might say that this is not optimal for traffic, but it might be optimal for good content.
That's a thing i hadn't really considered, but you're right, that might also discourage memes from forming. The old hubski users also seem to largely favor quality over quantity and see no problem with remaining a small site.