Love that you've mentioned this. We're actually looking at leveraging that filter bar to control all of the filtering options for the content list. What we've been thinking is a little different than your proposed design, but I think they address the same problems.
We were looking at moving the filters into the dropdown, so you can select a category or tag, choose document type, etc. As me add more filtering in the future, maybe "Documents I've created, Documents I've editied, Documents I've commented on, etc" those would be added to the filter bar drop down as well.
Sounds fantastic. When first hearing about the 4.0 filter/category features at JiveWorld I imagined them as drop-down menus. Though if you're going to have a menu for individual tags, I only wonder what happens if there are 100s of them...
While you are working on the filters, it will be really nice to have an option to display the documents in alphabetical order. Many users prefer to see a listing of documents in alpha order.