I think a lot of OT fans in particular view RotJ as the point when the franchise started to tip in a direction they didn’t like, with George apparently exerting a greater degree of creative control and leaving less room for collaboration. Some say that it marked the beginning of George “selling out.” Often, people back this up with comments from Gary Kurtz or Lawrence Kasdan, who were both clearly unhappy with the direction taken in the story, wanting a more downer ending that challenged the audience. George wanted a more clear-cut happy ending, of course, and I think that was the right call in the long run.
These days, I think people gravitate toward criticizing RotJ because out of the three films, it’s the one that’s generally regarded as having the most issues to pick on. It’s the “low-hanging fruit” of the OT that’s used to show that the trilogy isn’t perfect. I personally think most of the issues people point to are fairly surface-level stuff that doesn’t really damage the movie as a whole. A second Death Star makes sense in-universe. Han surviving was the right choice, though I wish he had gotten more to do as a character. Vader seeming out of character is consistent with his conflicted state of mind from the very end of ESB onward.
My main criticism of the film would be that the pacing drags on Endor at times. Besides that, I think it’s great.