You asked for it ...
Jabba's palace (and pretty much all of RotJ) is a muppet-fest.
It's not a reflection of the effects of the time, either. Remember this was 1983. Alien came out 5 years earlier in 1979 and Blade Runner came out in 1982. Muppet effects were already dated for many years.
Yoda in ESB was acceptable only because his character was so strong and I could suspend disbelief.
Even the Rancor and Sarlacc weren't scary at all. I was 17 when I watched it in the theater and thought the creatures in this film were just poorly done compared with the first two movies. The walking pig-caracatures with battleaxes? These were just D&D characters. I rolled my eyes and couldn't believe how bad the monsters were done. They looked just like guys in rubber suits. The way they were lit just made them seem like guys in rubber - which they were.
The number of muppet cameos just looked like a big merchandising grab. That was obvious to me even as a teenager.
ROTJ is rife with dated movie tropes. The explosive threat. The trap door to a monster-pit. The stick/bone used to keep a creature from biting down. And that's just in the beginning.
The style of humor changed entirely. The humor in both ANH and ESB was character-driven. But ROTJ simply descended into complete farce. e.g. crying Rancor keeper
Leia's bikini was just a shamelessly calculated attempt to provoke the teenage libido for those of my generation who had been smitten by the princess in the first film. Even as a teenager that was obvious to me and I was more offended than attracted. I thought SW was above this dumb exploitation. Apparently ROTJ wasn't.
I couldn't suspend enough disbelief to think Leia could choke out Jabba. That's like me trying to choke out an African elephant with a length of chain - not bloody likely.
RotJ wusses out in so many ways. Every male character is stripped of testosterone.
Han becomes an instant mushy Romeo overnight. He was still a scoundrel in ESB. There was none of that in ROTJ. Every sappy line was simply out of character. I don't blame Ford for giving a half hearted performance.
Boba Fett who was menacing before dies in a throwaway comic gag.
Stormtroopers used to be threatening. In ROTJ they become keystone cops. They fell for just about every stale Hollywood trope. They were literally a joke. It doesn't help that they were taken down easily by prehistoric teddy bears.
Vader is devoid of menace. Bad acting and bad dialogue. In ESB he still had conviction in but in ROTJ he's suddenly a tortured guy struggling to emote through a helm.
The Emperor looked laughable to me in 1983. ESB had him as a giant hologram for a reason. The way he was lit on ROTJ made him look like a sad guy with off the shelf makeup and bad teeth.
The camera work and editing is shoddy throughout. It's worse than amateur night at first year film school. When the Emperor is zapping Luke you get that hilariously amateurish shot of Vader turning his head as if he's watching a tennis match to demonstrate his conflict. It was as laughable as Vader going, "Noooooo" at the end of ROTS.
The effects weren't improved - there was just more of them. To me the they just cranked up the scale and number of effects but the execution was the poorest of the trilogy.
ROTJ violated the internal logic established in the earlier film in many ways.
ANH established that C3PO is "not good at telling stories" but in ROTJ he's giving an entire multimedia presentation to the Ewoks.
Obi Wan as a ghost sits on a corporeal log and has a long conversation. WTF? ESB and ANH show his passage to death as a state that's still mysterious. In ROTJ it's as if he never died.
It bothers me that an entire Super Star Destroyer can be taken out by a single kamikaze snub fighter.
Luke's character arc from ANH was his journey to follow the path to become a Jedi. The climax of that journey should be Luke taking on the Jedi mantle. That's what we've been looking towards since ANH... but that grand event happens offscreen somewhere between ESB and ROTJ. He appears in ROTJ suddenly a full fledged Jedi. He was still a novice at the end of ESB still controlled by his emotions... Where was his epiphany?
By the end of ESB Luke was still fumbling with his training and still dealing with failures. When he confronts the apparition of Vader in the cave, his face is in the helmet shows that his inner self is the true enemy. An enemy he has yet to conquer.
When he leaves Dagobah for Bespin he's still acting on emotion and it's against the warnings of Ben who says: "Only a fully trained Jedi Knight, with The Force as his ally, will conquer Vader and his Emperor. If you end your training now, if you choose the quick and easy path ... as Vader did ... you will become an agent of evil ... "
The two wisest characters (Ben and Yoda) of the series told him it's a mistake to leave. Were they wrong? Apparently so. Luke leaving to Bespin just proved that "taking the quick and easy path" was the correct decision. He's essentially making the same mistake as his father did.
So ROTJ just abandoned the themes established in ANH and ESB and focused on the soap opera.
There's more but this is just off the top of my head.
And I haven't even touched the Ewok issue...