I suggest that if a user searches while in a particular directory the search should by default use that as the parent directory that it searches under. I don't know if this is just me being opinionated and "Explorer did it this way, so should Nautilus", but to me this just feels 'right'.

Other information:
For example, if I was looking for an image, I would probably start off by browsing for it. Next, I would find a directory filled with images and would want to search for the one I was looking for. Finally, I would search and have to notice that all to subtle plus button, think about what criterium meant, on a whim click it, then navigate to the directory of images all over again.

(By the way, the add criterium button seems a lot like mystery meat, why not just have an advanced button (that is a different colour from the background) with all of the "criterium" on one panel. To me, that seems to be the convential way of feature hiding.)

I would love to hear other people's thoughts on this.