ROTJ is unfortunately quite poorly written and directed. Pacing is really bad. The opening segment has some cool ideas like the Sarlacc pit, but it's staging lets it down. The Dagobah section feels pointless; Luke visits just to see Yoda die. Speeder bikes are cool, but again, not that exciting to actually watch. The Ewok scenes cemented the incompetence of the Empire for all time. And the less said about the 'twins' twist, the better.
The one thing the movie really has going for it is the savagery of Luke vs Vader. Beyond that, it's mostly notable for being an immense anticlimax. Empire raised the stakes and made the world feel truly dangerous. Jedi proceeds to scrub away the weight and darkness it had established. Resolutions come too easy, and the characters have a forced chumminess to their interactions. Even Vader looks a bit cuddly under the mask.