Fifty reasons why Return of the Jedi sucks. Number one with a bullet is "Ewoks, Ewoks, Ewoks".
But aside from what we see onscreen, the Ewoks are miserable little creatures for a completely different reason: they are the single clearest example of Lucas' willingness to compromise the integrity of his Trilogy in favor of merchandising dollars. How intensely were the Ewoks marketed? Consider this: "Ewok" is a household word, despite the fact that it's never once spoken in the film.
When I was a kid, I had a friend who knew all the names of even the most minor characters from the Star Wars movies and had no idea where he got that information. Was there a fourth movie I didn't know about? It wasn't until much later that I realized his extensive collection of SW action figures had filled in all the blanks for him.
BTW, the current definition of an Ewok on Wikipedia reads:
Ewoks are a fictional species of teddy bear-like hunter-gatherers that inhabit the forest moon of Endor and Settlement operations at Goldman Sachs.
Goldman, you've been burned!