It's not that it wasn't dark. It's that it was so very childish, and again I mean that in a neutral way, it's a film meant to appeal to ten year olds, while the other films were the sort of films that adults make for themselves that also appeal to ten year olds; Spielberg was the king of that genre too, and I feel Pixar and Peter Jackson have been doing decent jobs of it too. If it was darker it would have helped it's rep, and I also think it's silly to say its dumb to expect the film to be dark, there was every reason to excpect it to be darker than it was, it's almost silly to have expected it to be as light and kid-friendly as it was, but that's not the root of the problem. The backlash came from the fact that Episode I wasn't a great film, it was always going to get mediocre reviews, but the fact that it was child-friendly by Return of the Jedi standards made things twice as bad and amplified the negativity that was already there. Really, Episode I isn't terrible, it just wasn't all that good, and people hadn't waited 16 years for something that was "meh." So, viewers were guilty of exaggerating things, but the film was guilty of being a bit lousy as well. Put those together and you have the cinematic disappointment of a lifetime--sort of unfairly earned, but also unable to really defend itself because it's sorta poor in the first place.