Most people follow fashions, knowingly or unknowningly. The reason I say this is a trendy viewpont because back in the old days Return of the Jedi didn't receive much criticism from the audience; it was nearly the same widely adored as the previous two films. The critics were less satisfied, but due to their roles they always enlarge the problems and with each sequel they're more and more critical anyway. Meanwhile the internet took over the world (and the prequel trilogy started off) the fans got wider and wider community to talk to and slowly they've started to put on the critic attitude, overanalyzing problems they didn't felt obtrusive originally. This who-is-more-insightful competition lead to the present state, when it is cool thing to bash ROTJ and blur it together with the prequels. Once again, the loss of credibility for me here is that ROTJ haven't been heavily critisized by the fans for a very-very long time.

Also, this Vader-shouldn't-be-Luke's-father thing... it's just as suspicious. It was an unexpected twist, for God's sake. It had a very important role in the impact ESB had. It is evident nowadays because we've seen it a hundred times in ESB and in many other movies too since then. And some of you would now change it. Could someone come up with an idea remotely as interesting as this was back in the day? Common sense, people! It was one of the moments that made ESB what it was.