Image zooming is painfully slow, especially since the image disappears while it is resized.  This is exceedingly annoying when zooming in to large images.  Ristretto should also filter out images that it cannot display (such as cr2).  It's quite annoying to go through a folder of pictures and have every other one be blank.  Other viewers like eye of gnome just skip images they can't display.  

Also, the fact that the 'open folder' button does not start in the same folder as the currently opened image is quite inconvenient and would probably be a one line fix.