The original trilogy were good movies, but they weren't amazing movies. However, they were incredibly groundbreaking movies. There had been almost nothing like it before in terms of special effects and pacing. It earned a special place in our culture by being an unforgettable experience for a generation. To anybody watching them for the first time now, they're just an alright story in a good setting, and the presentation was well-executed in most places where it mattered.
The second trilogy came out at after flashy effects and action movie pacing were completely normalized. The people going to see it weren't very conscious of that. They held onto those expectations that were built up by an artifact of timing. What they found were movies forced to stand on the same merits as any other, and they didn't succeed very well at that. So... huge disappointment and backlash.
And some of the stuff they did was just incredibly stupid. Jar Jar Binks was 100x more annoying than C3PO ever was, and was treated as much a main character as the Jedi. The pod race deserved to be 1/4 as long as it was. And... mitichlorians. Fucking mitichlorians. Worst idea ever. EVER. When they had that conversation about Anakin's mitichlorian counts, I was in pure shock that they would do that. At that point, I was like "Umm... ok... just... just give me a bunch of lightsaber fights and I'll be happy... ok? Stop trying to do anything else... just lightsaber fights." Coincidentally, I was happier with Ep 2 & 3.
The best work to come out of the entire Star Wars franchise, in my opinion, would be Genndy Tartakosvky's Clone Wars mini-series. Blows all the movies away, even the old trilogy.