I think Leia being his sister did serve a decent plot purpose- it gave Vader in reading Luke's emotions a way to manipulate him into a rage. That has always been a very powerful scene for me, being a protective brother as well.
I think people really don't like it because it makes the nature of their relationship a little incestuous.
I always think of the kisses between them being Marty + Loraine (back to the future) type kisses.
I don't know if there is a name for it, but I think psychologically brothers and sisters can often have something similar to an oedipus or electra complex.
I like ROTJ. I think it works well with ESB. Lawrence Kasdan was still there, Marcia Lucas was still there.
With the Ewoks, I'm not like "oh man, I can't watch this anymore" like I am with Jar-Jar. Over the whole charred corpses of Owen and Beru to Ewoks- there are dramatic ewok death scenes as the tide starts to turn in the battle during one portion. Also you have the darkness of Vader's death and Luke's cremation of him.
And I for one like the Ewok song at the end.