The idea to replace gThumb with Ristretto has been proposed in the Roadmap for XUbuntu 12.04 Precise Pangolin. The main advantage of Ristretto is that "It is strictly an image viewer and it does nothing more than that."
Ristretto is a fast and lightweight picture-viewer for the Xfce desktop environment. It has gained popularity among the users of XUbuntu, which uses XFCE desktop environment, for its speed and performance. Developer Stephen Arts says in one of his comments "While ristretto is primarily developed for viewing images, and I do not want ristretto to fool around with the contents of images – other apps like gimp are a lot better in that then ristretto ever can be. "
Ristretto, unlike other image viewers, does what it says. You can navigate between images, view image thumbnails, run a slideshow, flip or rotate images. Ristretto does not do any basic image editing. But it is aware of the image editing applications and gives you options to open the images for editing in apps like GIMP.
With Ristretto being actively developed, we can expect a few more features implemented through a plugin mechanism. Stephen Arts says in his comment "One possibility to extend ristretto’s functionality in this direction is through some kind of plugin mechanism (adding menu-items), or custom-actions (like thunar’s, but then tailored for images) that can be used to call ImageMagick’s convert tool."