I just checked the website rotten tomatoes, which evalutes movies based on both audience and critic feedback. Their "cutoff" between a good movie and a bad movie is 60% approval.
The Phantom Menace got a 57% approval rating from critics and a 62% approval rating from fans. This is a "meh" grade.
Attack of the Clones got a 67% approval rating from critics and a 66% approval rating from fans. This is "meh" to "ok."
Revenge of the Sith got an 80% approval rating from critics and a 65% approval rating from fans. This is "ok" to "good."
I think in the seven years since Revenge of the Sith, more and more people have bought into the views of the detractors, many of whom proclaim that the prequels "destroyed their childhood memories" and "Lucas ruined Star Wars," etc., etc. Since most people who like the movies only like them moderately--and those who dislike the movies have stronger feelings--there isn't much of a debate (because supporters don't want to get into the argument)
So, on message boards, conventions, comic-cons, etc. one usually only hears one side of the debate and the "hard-core" fans begin to buy into the "hatred hypothesis." Thus, even though a majority of fans and critics liked all of the prequels, the geek community's group-think has deemed them totally awful.