Nostalgia certainly plays into it. For me, films I saw as a child I watch differently now than those that I didn't watch in my younger years. It's like my brain has two modes of viewing - pure childish marvel, and mature critical analysis. The real way to describe this would be passive vs. active viewing. Nowadays I tend to watch most movies the latter way. Of course, when I see a movie like, for instance, Thor: The Dark World, I turn off the more analytical aspect. I let the movie take me for a ride, but I still watch actively.
When I watch something like ROTJ, I find it's much easier to slip into passive viewing, because I've seen the film that way so many times. It's not how I prefer to watch movies, and I feel movies should stand up to active viewing as well, but I can appreciate a film that can simply give you an experience. ROTJ is up there with the other two when watched passively, no doubt. But when watched actively, there leaves much to be desired, unfortunately. That is why it is the least of the three.
Unlike most here, I was a child when the PT was released too. I greatly enjoyed those films at the time (of course there were some things about them that were annoying even for a kid). When I watch them actively today, I can't fail to notice the numerous flaws. However, I think I'm luck in that I can easily slip into that passive mode, and actually still enjoy them in their original state.
I have to say because of this I feel I'm actually more objective in my active assessment of the PT than most. I don't not share the hatred of them that most do. I think when people watch them, it is easy for them to slip into viewing the films with nothing but hatred. But I can say there are in fact redeeming qualities the the movies. They're not that bad. TPM and AOTC aren't really all that good, but ROTS is. I think it's closer in quality to ROTJ than most realize/would like to admit.