ESB was dark because Lucas wasn't directing. Had he been, Cloud City would have a carnival in the middle of it and Hoth would have been inhabited by not only wampas but fuzzy snow bunnies that would watch from a distance and do cute things while Luke's unconsious body was dragged away.

http://www.saskschools.ca/~gregory/arctic/animals/ahare.jpg

ROTJ was more childlike because Lucas was directing again.

ANH was the movie of the summer. It was a stand alone story that everyone and their grandma could enjoy. ESB and ROTJ appealed to the people who became fans of the first movie.