It seems like this community is full of people who want to work, and want to learn. Naturally, that doesn't lead to very many sociable people.
I think a great way that people could encourage cross-sketchbook discussion at least, is not to put up your own sketchbook to learn and other sketchbooks to comment, but remembering that you can use somebody else's sketchbook to learn for yourself as well.
Say you're great with lighting, but terrible with form, but you've noticed that someone else is just the opposite - they've absolutely nailed their form, and you can't help but think, 'wow.. how can I do that?' and so, you ask! Sketchbooks of other people are as much of a place to discuss fundamentals as the critique subforum or the art questions area. Because every sketchbook is a journal, every post can be a prompt for an in-depth discussion for some subject, or some technique.
You can even ask someone about the excercises they're doing, and in all likelyhood they'll be happy to tell you, because you've done them the service of bumping their thread.
I thought, perhaps falsely, that superpros were necessary to drive a community for learning, but honestly if several people get together and plug each other's weaknesses it wouldn't be too much time until the group as a whole improves. And that attitude would probably encourage the egalitarian learning circles that we want on this forum.