Dankster, the lumping together of all drugs is absofuckinglutely the biggest hurdle to get over when talking to people about legalization of weed. "Well, it's drugs and drugs is bad!"
I think the propaganda machine has done this very consciously and deliberately. Dumbing down their target audience with sly lies. Even DEA mouthpieces, who theoretically have some degree of brain power, spout this crap. And try to get an honest answer out of a legislator; they always say, in response to marijuana policy questions, something about not supporting legalization of "drugs."
Also, as racerboy said, crap propaganda programs like DARE tell kids that all drugs are the same, so naturally some will try the bad shit when they find out weed's OK. I believe that the psychos who sponsor and design these programs do so to CREATE the so-called gateway effect. They consciously and deliberately create increased addiction rates to support their otherwise unsupportable claims.
Maybe that's just my tinfoil hat talking, but not all of these people are ignorant idiots. At least not in the sense that they're unaware of reality. They're just busily creating justification for their own jobs and, more importantly, their power over you and me.