Huh. You figure that the lack of negative reinforcement is going to draw the downvote-shy lurkers out in a big way? I always figured those are the same folks who lurk 4chan, browse SA, and never participate much anywhere else - the "fear of downvotes" is just an excuse. There are almost always going to be "silent majority" readers, I'm not sure if the exclusively positive-reinforcement model is going to change that where other systems with no negative reinforcement at all have failed. I dunno, mostly just musing, but I'm not sure that it's such a make-it feature or trait.
You are certainly correct in your observations of the "lurker" crowd. No matter the site mechanics, there will always be a large component of site users that browse content aggregators for, well, content, rather than the community. However, for those who are a part of the larger reddit community and (as amusing as it is to say this) actually care about karma points, Hubski offers an alluring system to quantify social acceptance. Users only need 8 dots to earn a badge, and with no possibility of being "downdotted", earning badges seems to be an easier goal to accomplish than racking up thousands of karma points. Hubski offers a crisp, customizable content feed that people who like reddit will find familiar. With the comparative easiness of racking up badges, Hubski will likely attract lurkers and avid users from reddit alike.