No doubt there is a lot of cool stuff in the prequels.
The problems they suffer from are casting in some cases, dialogue in others and far too much CGI - including the abomination known as Jar Jar Binks. The stories are interesting and some scenes are amazing. Some casting is brilliant (McGregor, Portman, Neeson) but the dialogue, limitations by other bad casting etc keep them from really carrying those movies like they could have.
I think the reason people criticize the sequels so much boils down to them being disappointing over time. I think people liked them ok when they came out but over time, the quality of 4,5 and even 6 if you forgive the ewoks, has held up and become even more legendary. 1,2 & 3 weren't special and just seem like average sci fi movies now.
7 is old school and done right.