My problems with ROTJ:

Jabba the Hut

Don't get me wrong I like Jabba and it is important that we finally see him in the last movie. But the entire act felt like a movie within a movie. Its sole purpose was to get Han back to the group. Yes, in Star Wars there are the scenes with C3PO and R2D2 at the beginning and in ESB we got the battle at Hoth. But those scenes also established the main plot. They showed us the Empire and set the tone and main conflict. In ROTJ the entire first 40 minutes are just about saving Han and did not further contribute to the main plot. As a result the actual plot felt rushed.

Lack of dilema at the crisis moment of the movie

The crisis moment of the movie is when Luke had to choose whether he should kill the emporer and join the dark force or resist and stay true to his own believe of the force. This is no real dilemma, because it's a choice between good and bad. Since Luke is a good guy, there is no real question which one he will choose. Dilemmas require a desicion between two equally bad or equally good things.
Like in Star Wars when they were attacking the Death Star and Luke had to choose between using the technolgy of his x-wing or using the force. It's a choice between two equally things, because on the one hand his x-wing already failed to hit the Death Star and on the other hand he doesn't really know if this force thing will work out or not.
In ESB they complicated the crisis moment even more. When Vader revealed that he was Lukes father, Luke had to choose between joining the dark side or his believe in the force and between killing himself or survival. Note that these two desicions are again just plain simple desicions between good and bad. But since staying true the force means killing himself and survival requires joining the dark side, the combination of these two decisions is a true and compelling dilemma. <br />Why is it so bad that there is no real dilemma in ROTJ? Well, when we know which desicion the protagonist will take, than there is no tension in the movie. Which brings me to the next point.

Lack of tension and increase of complication.

What's so good about ESB is that they managed to further complicate the story of Luke becoming a jedi by just by telling us that Vader is his father, thus creating the most personal obstacle for Luke to overcome.
In ROTJ they not only failed to increase the complication, but also to keep the tension at the same level as in ESB. Just look at the Death Star II. We have already seen in Star Wars that our heros are able to inflitrate the Empire and to destroy there main base. There is no point in showing us the same thing again because we already expect the Rebels to win. Was there anyone wo really thought they won't destroy the Death Star this time? The same thing is true for the Battle at Endor. We have seen it before how Han, Leia and Chewie outthink some Stormtroppers and this time they had support by a army of bears thus decreasing the tension even more. Sure the Empire had some robots but it was not like they had to face a army like at the beginning of ESB.

Too much focus on Darth Vader

The movie also suffers from making Vader more important than he is. For example how the relationship between Luke and Vader is handled. During the confrontation scene at Endor Luke seems more mature than Vader. In fact Vader is the one who is more effected by their relationship. This draws to much attention to Vader because Vader being Lukes father is something that should complicate the life of the protagonist not the antagonist.
But what really hurts Lukes story arc is the fact that he got saved because of Vader. Vader is the Deus Ex Machina for Luke. Vader is the one who must take a desicion at the climax of the story, which is wrong because the story is about Luke and not Vader. He is the one who has to face a real dilemma because he must decide between staying loyal to the emporer and killing his son or saving his son but betray the dark side. For Vader this is a dilemma, because up to this point the dark side was something good for him. That Vader has the last saying is just wrong from a storytelling perspective. It's Lukes story so he must take the desicion at the storys most suspensful point. He did it in Star Wars when he had to decide between technology and the force. He did it in ESB when he had to decide between joining the dark side or killing himself. But he hadn't to choose at the climax of ROTJ that not only should bring closure to the movie but also to his story arc of the entire OT? That makes no sense.

It doesn't feel like Star Wars.

Okay this is a personal point but to me ROTJ just doesn feel like a Star Wars movie. This is because there is a tone shift to give it his own feeling. ESB also has this tone shift but it still feels like a Star Wars movie. Two thing of the tone in ROTJ that bother me. It is too sexual and it has too many wierd aliens. Before you call me prude, there is just one thing that is too sexual for me and that is Leias dress at the beginning. It just does't feel right to me because this revealing dress draws associations that just don't belong to Star Wars. Star Wars was always some fairy tale in space and not a celebration of dirty fantasies. I'm just speaking of the OT of course.
Another thing are the many different creatures. Star Wars and ESB had aliens too but they were just in the background and said some one liners. In ROTJ the entire first 40 minute is full of aliens. And after that we got the Rebels who all of the sudden got more aliens than before. My point is that in ROTJ there are so many aliens that they are drawing to much attention to themself. You know in Star Wars and ESB there were just enough aliens to tell us that there is a greater universe going on. In ROTJ they shoved so many creatures in the movie that I started to ask myself why these things are in the movie, what is their contribution the the plot?