One of the problems I've always had with RotJ is Endor. It was shot in a national park, and it kinda' shows. There doesn't seem to be anything threatening about it as an environment. Even Luke's old home, Tatooine, has the potential to be more threatening. Although it looks quite creepy at the landing platform with Vader at night, the rest of the time it seems as though the rebels are a hiking team rather than people infiltrating a strange world.
There must be some way to make it seem like the kind of exotic environment we're used to seeing in Star Wars - maybe a fog effect could be used or a shot of a very distant canopy, emphasising unearthly massive trees, which then pans down to show the rebels.
Any other ideas?