I've written recently about desktop search and YDS: Faughnan's Tech: Yahoo! Desktop (X1) is the new champion.
As I noted there and elsewhere, metadata is one of the big missing pieces to desktop document management and search. Not every document I save is of equal value. Some are "five star" documents I want to find easily, others are of less value. Current file systems don't track this data for files. This data is tracked to a certain extent in Gmail (zero to 1 star), iPhoto/iTunes (5 star) and Outlook emails (3 star: low, norm, high).
I'd like my desktop OS to allow me to specify a "value rating" when I store a document, and I'd like to use that star rating as a criterion when I search and sort.