The standard Swing file choosers (see SwingSet2 FileChooser demo, Plain JFileChooser, Preview JFileChooser, and Custom JFileChooser) aren't quite matches for the stock GTK+ versions (see File->Open in gedit).  The "New Folder", "Delete File" and "Rename File" buttons are too small (the stock ones have more space around the text).  More critically, the JPopup showing the current directory has a white background when it should be grey, has only a single down arrow when it should be a combination up/down arrow, and has a scrollbar when there shouldn't be one.