I know this is more to do with ROTJ than ESB, but since it's on the topic of adding 3D to matte paintings, I've always been very annoyed with how the emperor's death looks--
This shot would look way better if the matte was on a 3D mesh and the camera was given a bit of movement-- not to mention that Palpy's lightning looks like crap, and the explosion at the end could stand to be much better if it were re-done.