The simple fact is that there is no evidence that GMOs, as a blanket group, are dangerous.
There’s also another reason that GMOs aren’t considered dangerous: decades of scientific research support their safety. As Pamela Ronald, a UC-Davis plant geneticist, phrased it last year in Scientific American: “There is broad scientiﬁc consensus that genetically engineered crops currently on the market are safe to eat. After 14 years of cultivation and a cumulative total of 2 billion acres planted, no adverse health or environmental effects have resulted from commercialization of genetically engineered crops.”
Update: Mark Bittman has more on Prop 37 and related food labeling/control initiatives that were defeated. He agrees that Prop. 37 backers made errors, even egregious ones, but said that it wouldn't have mattered because Big Ag was determined to defeat ANY labeling issue.
And, he's right -- labeling means control, and that's the last thing Big Ag wants to let go of. Too bad "scientists," even if they think that Monsanto has ... oh, a few problems ... don't recognize the issue is far more serious.