The text input control (top right) should retain its value when used. Thus making it much easier to modify the search.
Currently one has to completely re-enter (with all typo likelihood), or navigate back (thus losing the initial results as reference.
EDIT: JUst tried again on SF. Search for "[windows]" consistently left a blank search box... some other searches left it populated and some didn't. There seems to be a lack on consistency. (All in FF.)
EDIT #2: As noted in the answer below if the search consists of just tags, then it is treated as selecting those tags. This should still leave the search box populated to allow refinement of the search (i.e. keep the page content consistent with a search).