Aside from those you listed, I also wouldn't say Carrie Fisher was horrible either (though definitely not great). Maybe Han and Leia simply had the best dialogue, but their scenes together were pretty much the most memorable across all three films until it was Vader with Luke. Mark Hamill wasn't "good", but I'm not sure you could actually call him "horrible". If you want to be honest, he came off like someone inexperienced, but he did progress over the films, and even if no one gave an oscar worthy performance, pretty much all of the main characters were better than everyone in the prequels other than Ewan McGregor and to a lessor extent Ian McDiarmid. It's not nostalgia or super fan bias or some shit either. Their performances were not stellar in the OT, but on average, the delivery was simply much better overall. Could have been many more factors besides dialogue (because he had co-authors, or people that actually wrote the majority of the screenplay for him) and a lack of green screens, but it doesn't really matter what the cause was.
You can have your own theory as to why the masses love to "circle jerk" on the prequels, but like I've said before, you can't just generalize and claim to know the reasons behind why anyone thinks something. None of what you said above much applies to me, or the friends who I actually went to the theater with to see them, nor my relatives that cared much about the franchise. Or rather, those reasons that are actually applicable, hold only the smallest relevance compared to much larger criticisms.
It might apply to some people, but it probably applies to a lot less than you might think. Many people might have had a vague imagining about how they expected the films to be, but with or without expectations, they simply were not good movies. That's totally disregarding them being Star Wars movies too. They all suffer the same thing the majority of expensive, super spectacle films have since the late 90s: too much focus on effects at the cost of everything else. Besides, if you've followed the hindsight discussions or documentaries with Lucas, when he decided to even make the prequels, he had absolutely nothing written and had no idea where he wanted to go with them, so he pretty much winged it as he went along. During the filming process, he had total control and was surrounded by yes men, instead of those that could have greatly helped to make the movies better.
I do somewhat agree that the lack of mystery might have made the movies seem worse than they were, but I don't entirely agree. I think Lucas killing off some of the mystery that made the OT so special hurt a lot, and I also think that there was a hell of a lot of interesting branches he could have taken with the prequels that no one really thought about, regarding things only hinted at in the OT. What did he do though? Write about the most boring shit ever, and the scenes that had the most relevance to the OT (such as the fall of Anakin) were horribly rushed and horribly edited. "I don't like sand. It's coarse and rough, and irritating it gets everywhere, kind of like my dialogue. Time to kill some kids!".
