>is it better off being comprehensive (e.g., all the libraries, all >the archives, all the musuems, etc.) to have as large a pool as >possible, or whether having a bunch of more focused fora (e.g., the >potential digital preservation one mentioned by Trevor) is the way >to go.

Anyone interested in this question might want to check out this thread on the distinction/reasons for thinking about doing a general libraries Q&A site and one focused on digital preservation/stewardship http://anjackson.github.io/zombse/062013%20Libraries%20&%20Information%20Science%20Meta/static/questions/50.html (thanks to Andy Jackson for the Zombie version of the site he threw up so we can read these discussions.) 

My short recap on this is: I think a general Libraries discussion site is likely to be so broad that it might not work (ex. some folks in the LIS Stack Exchange wanted to talk about things like book suggestions for young readers, and best practices for weeding book shelves at public libraries.) With that said, the digital preservation focus might be more spesific than this kind of thing would need to be. So if it was digital stewardship/curation/etc it might have a section or channel focused on digital preservation, on digital access, on born digital collection development, on digitization, on training and education, etc.