It would be very useful (at least to me) to support multiple terms in the "Filter Subjects" box when searching/browsing. For example, I might like to be able to exclude any entry with either "German" or "French" in the subject, seeing as my German is really poor and my French is non-existent. (And it would be good to keep a bunch of previous terms sets around so I don't have to retype them every time).
I don't think there's a strong need for multiple include terms as most search engines seem to do that already (although IIRC some treat them as connected by AND and others by OR, so it could be useful).