TFA had bad pacing. The action sequences were so packed in that it was exhausting in parts. The plot was driven by happenstance. The characters weren't developed well. Side characters and key plot elements were introduced in a haphazard, confusing way. The villains were lame and underutilized. The big threatening space station didn't seem all that threatening, and the main characters didn't seem to find destroying it all that difficult. We were never given any reason to care about the people the superweapon killed. The movie was, like, turbo-dark; there were a couple of light-hearted moments, mostly involving BB-8, but they were short and always bookended by more sad-dark.
In terms of character development, TFA felt less like a story, and more like speed-dating, where the hero of the next movie is whoever sells the most action figures. Will it be Rey? Cowardly Stormtrooper? Maybe... Shiny Stormtrooper??? Ooh, I'm super excited to find out!
RotJ wasn't a perfect movie. It had problems, but they were different problems than TFA, and imo less acute ones. The Ewoks were just... way, way too much. But the pacing and editing are pretty much legendary: the last third of the movie is like one big action sequence spread across three subplots, but the way the movie alternates between them breaks the tension perfectly. Its plot was driven by the planned and deliberate action of the characters. World-building was kept light, and characters were never introduced abruptly. The villains are among the most iconic in movie history, precisely because they were used so well in that movie. The Death Star was super threatening and was picking off 'good guy' ships, so you could easily understand the harm it was doing. There were lots of light-hearted moments (maybe too many, c/o Ewoks). Like, RotJ was still a kids movie, and I definitely don't enjoy it as much as I did when I was younger, but it's pretty firmly in the cinematic canon, and Episode 7 will pretty firmly never be.
By the way, I absolutely disagree that it was a rehash of ANH. Broad strokes, TFA and ANH both featured a superweapon, and both stories were structured in a hyper-traditional hero's journey kind of way, but the similarities end there. Distilled, ANH is a standard "peasant leaves to rescue a princess, saves her kingdom, returns a knight" story. TFA intentionally runs at right angles to those kinds of primal stories. I think it's one of the (many) reasons TFA has a weak plot, actually.