This is not speaking for everyone that thinks there's a bit of a dip in rotj, just myself. It's a fine movie, but it has a few issues that could be looked at as the small cracks showing in the franchise that would become bigger problems later.

A big one is they reused a few beats from the first movie. Start at Tatooine, deal with the death Star, destroy it. Kind of explored territory, similar setup to the first movie. Honestly not too bad in and of itself, but the franchise really leans in to callbacks and reusing narrative structure, and this is where it starts.

Character motivation and writing is a little shaky in places. It's enough to hold the movie together, but it was better in the first two movies. Stuff usually just kind of happens to the characters, rather than the characters having active agency in the story. Villains become non-threatening and incompetent, hell even storm troopers start turning into the useless cannon fodder they're known for now. The characters are overly hammy, and the line readings are wooden at times. This goes a bit back end forth with the franchise, but it is a noticeable downgrade compared to the first two movies. Especially with how good Empire was with this sort of thing, it stings a bit. This goes on to be a huge issue with the prequel trilogy, on and off in the sequels as well.

Luke and Leia being twins is the first example of the plot event that happens because they wanted drama and a big reveal, but didn't set it up ahead of time. It's not hugely bad, but it's a bit why go in that direction? It kills the Luke/Leia thing that the last two movies have been building on, I assume so everything is clean for the ending, but it's a bit much of a jump when they sort of made out last movie. The lack of planning is something that bites them in the backside pretty hard with the sequels, but it shows up here first.

The last one that's weirdly specific, but Yoda wasn't supposed to talk like that. He is putting on an act in Empire, up until he's found out, instantly drops it, and doesn't talk like that for the remained of the movie. When he turns up in Jedi again, he starts talking in the fake voice he was using, and now he's forever stuck in that way of speaking. It's iconic now, but it's a little weird continuity wise.

There's a lot of pretty decent stuff in the movie, but compared to the genre defining first and amazing second, it's just pretty alright. Which is fine, but it's hard for me to shake the feeling that it's the beginning of the series solidifying into a slow decline. Most things go that way, and I don't have strong feelings about the series these days, I more just find it fascinating how you can see the momentum of a cultural touchstone progress from movie to movie.