I like a tagging system, whereby each question is sorted into a few general groups - "algebra", "calculus", "prealgebra", etc. in the math group, for example - and the user given the choice of seeing the unfiltered groups (i.e., all questions), or just a subset of the questions.
I think it would also be a good idea for individuals to be able to create their own tags - for example, "Elliott's studygroup" - so then a circle of friends could make their own study group without necessarily having to create an entirely new forum.
Of course the trade off is the ease with which OpenStudy can be used and learned by new users. Possibly the intro material to the site like the little blue floating bubbles would have to be augmented. And there might be issues involving implementation and how easily it can be programmed about which I know nothing.