If it's an opaqueness restriction with the API or something, I'd like to suggest letting us at least combine the provided ones, so I could do something like (4-20) && (> 20) to get "over 4 minutes" which doesn't exist on Youtube but seems pretty useful.
Another thing that would be useful is filter-by-channel since the search function within Youtube for searching a channel's uploads (using the search button on a channel's page) is a significantly nerfed version of their usual search function.