I think you hit on some good points here.
Slashdot is a totally different environment than a professional setting: there are CEOs, engineers, high school kids, lawyers, etc.. all here posting their thoughts. They all get lumped into the same bin of comments and moderated without regard to those unseen traits (at least, in theory). One day I might mod someone +1 insightful and the next day -1 troll. I don't risk losing my job by doing so. No one opinion is higher than the others, so there's nobody to target with bribes (well, other than the people selecting the stories to comment on). I'm sure there are groups on /. that moderate certain opinions down which is an issue. Still, I think this site is pointed in the right direction at least.