> Do you have the same list of forum categories as before, named identically?
We had about 100 forum categories and sub-categories and sub-sub-categories. I condensed this to about 80 by rolling some of the sub-sub-categories up a level, and 301 redirected them to their parents.
> Are you linking from your home page or laterally through your site as you were before?
No. The forum index that currently exists on the sidebar of the homepage used to be on a different index page that no longer exists. That page was 301 redirected to the homepage.
> If your internal link structure has changed, you could be pushing less PageRank into your most important pages which is causing a drop in rankings across the board.
It *has* changed, but in a way that I think is for the better, as far as usability is concerned. I was expecting there to be a delay while Google recrawled the entire site, but I wasn't expecting *THIS*! We've lost 50% of our traffic at this point and it's still going downhill every single day.