I hardly use file managers, but some of these points seem quite interesting, I dislike the idea of hiding the search button when you click in the middle of the search field though, this will most likely only make it confusing for the user. A better options is to disable the search button when the user interacts in the middle of the search field(I think that's possible in GTK, isen't it?).

EDIT: Ignore my comment above, I missunderstood the part about the search button in the article.