I would like this with the addition of some sort of query builder, which would:

Display all the tags in some sort of sensible, sortable fashion
Take into account corner cases like me (who has a bajillionty tags, imported from LJ -- I swear I'm trimming them down, but it's slow!)
Display all the security levels in a way that takes into account who you are and what you can see (for my own account, I'd see public, access, private, and all the filters; for an account where I'm in the circle, I'd see public and access, and access would silently include filters I'm on; for a community where I'm admin: public, access, admins; community member: public, access; non-circled: public)
Let you select or type in the elements you want to include, in the way you want to include them.
Allow full Boolean logic.
Spit out the query when you hit go.


I'm actually slightly against having the output be URL-based simply because a full Boolean query with an arbitrary number of arguments could possibly exceed the allowable URL length.

Apply appropriate account-based restrictions in order to protect performance. (Because I expect this might be one hungry search.)