For what it's worth (and it's 3 AM here, and I've worked all day, and I don't know why, but posting in forums is a great post-work exercise for me), I have to come to the argument of story structure again.
ROTJ is imbalanced. The entire half-hour spent on Jabba, however interesting in itself, basically serves to get the storyline back to square one -- to return all our characters to that happy place of camaraderie where we left them back at the end of ANH. Cue much friendly backslapping, "Who's coming with me to our Next Great Adventure?" "I am!" "I am!" "And I am, too!" and here we go again, and against yet another Death Star to boot.
It's only after the first hour (and three mentor/apprentice discussions that follow on the heels of one another, making the post-Jabba part of the movie come to a dynamic standstill) that we get to Endor, and the proper storytelling, whatever you might think of Ewoks. You get the multiple, intertwined storylines of the ground battle, space battle, and the battle for the Skywalker souls, and it's all good. But the movie took its time getting there, and the way this storyline was achieved wasn't exactly ingenious.
(The Jabba bit might be integrated a bit more skillfully if the rumored scene of Vader's mental conversation with Luke is reinstated at the beginning of the film. So far, all we have to go on is the cue John Williams wrote for it, available on all the post-1977 reissues of the complete score CD.)
ROTS, on the other hand, also begins with an almost self-contained action sequence, but this one actually sets up all the plotlines of the movie to follow. It, also, proceeds to feature a brief lull in the action with the successive Coruscant scenes, but they tighten the conflict even further, and serve as a bit of a respite from the mad capers that preceded them. And then -- after Yoda gets to Kashyyyk, Obi-Wan arrives at Utapau, and Anakin learns the true identity of Palpatine -- we get the most sustained case of parallel storytelling in the entire career of George Lucas, and for the most part, he pulls it off so masterfully that I have no problem calling this his greatest directorial achievement yet.
His early films -- student experimental shorts, THX 1138 and especially American Grafitti -- indicated his proclivity towards this kind of editorial juxtapositions and intertwined narratives. But never before has he managed to have each of these storylines be as resonant visually and emotionally, and never before has he been able to structure them to play with and against each other in so many exciting and satisfying ways.
Be it the overall structure, or a layered sequence like Order 66, or a contrasted case of parallel editing like the birth of Vader/birth of the Twins, ROTS offers the most mature direction in the career of George Lucas. It feels like a well-tuned orchestra performance of a grand symphonic movement -- which is all the more surprising in the light of the uncomfortably disjointed AOTC, its immediate predecessor.