In the search field, when I type in "becker" and then change to search type to "user" nothing happens. It should automatically start searching using the user category. On top of that, there is no autofocus on the search field. Thus, I click on the search bar to hit enter and the typed in "becker" disappears.
So I would suggest three features:
when changing search type, automatically search again using the current expression in the search field
focus on the search field when changing search type and select the whole search expression (this way you can either type right away or keep the old expression by hitting the arrow button)
always focus in seach field when loading BibSonomy main view