I wish the Ewoks were the only thing that was wrong with Return of the Jedi. The Jabba the Hutt sequence had a bit too much humour and slapstick to it, especially since Jabba was from the very beginning portrayed as a vicious gangster. Though the character itself and his appearance was so striking it has made its way into pop culture, the whole scene was too comical for its own good. It failed to portray the seriousness of the situation that Luke, Han, and Leia were in. Not only was I certain that everyone was going to get out alive, it would have a melodramatic climax to it all.
Gary Kurtz rightfully criticised the inclusion of a second Death Star. (For more info, check this link: http://www.ign.com/articles/2002/11/11/an-interview-with-gary-kurtz ) Kurtz was Lucas' original co-writer and had more of an influence on Empire than Lucas did. Lucas in fact had a nervous breakdown during Empire and threatened to scrap the whole thing because he thought it was boring. Kurtz was raised a Mormon, but left and dabbled in other religions, particularly eastern ones. He came up with the entire mythology of the Force, while Lucas envisioned Luke as more of a cowboy/renegade type character. He finally split with Lucas after Empire over disagreements with the Return of the Jedi script. Han Solo was originally to be killed in the middle of the movie in a raid on an Imperial base, and the movie was to end on an ambiguous note with Luke heading off into the unknown, and Leia struggling with Han's death and her newfound leadership responsibilities. Kurtz reveals in the interview that Lucas decided to change the script to a more happier ending with Han still alive due to merchandising reasons. More toys to be sold. Leia was not to be Luke's sister, and the Emperor was only to make his appearance in a later sequel, as well as Luke's real sister.
The penultumate scene with Vader, Luke and the Emperor was the only thing that salvaged Return of the Jedi for me. It is there that you realise the helplessness of Luke's situation as he is revealed to be no match for the Emperor. Vader redeems himself, saves Luke, and brings it to the proper conclusion. I have a nagging suspicion that Lucas had help on that sequence (or perhaps was a remnant of what Kurtz originally wrote), because it's a little too deep for Lucas' style. If George had his way, one of the Ewoks might have taken out the Emperor by mistake. It's easy to see how much control Lucas had by the end of Return of the Jedi that hardly anyone has the gumption to challenge him on anything. From here he went into the Special Editions, and then the prequels. One can only hope that Disney brings in a talented team of writers and conceptualists. Lucas strikes me as the type of man who can come up with a nice abstract idea and bizarre concept, but is lacking in screenwriting and directing skills.