@netmeg - I think you just touched on something that makes this more clear for me.
What if it's a type of page that has a high exit rate when it isn't supposed to that is a factor for Panda?
I'll use my site structure (content site) as an example.
Homepage -> Categories -> Content
The content pages should have the highest exit rate. That's the meat of the site and what most people are there for.
The categories should have a much lower exit rate. They aren't the content that the visitor is there for - they should encourage the visitor to keep pursuing what they are looking for.
The homepage should have the lowest exit rate.
If google has categorized your pages (which I'm sure it has) it probably has also determined what sort of action the user should take on that page, whether it's following a link or exiting the site.
My site was hit by Panda on primarily the category pages. So now I'm off to see what percent of our category pages have a high exit rate. There may be something to this theory. I doubt it's the only factor, but I think it could shed some light on things at least for me since I knew even before Panda that my category pages needed improving.