|Freakonomics did an interesting podcast about alcohol and marijuana, and specifically which is more dangerous. The general view of economists is that alcohol has an enormous societal cost by way of addiction, drunk driving, and crime, while marijuana doesn't. It's an interesting thought experiment to consider which would be accepted and which would be outlawed if we had no knowledge about them and they were discovered tomorrow.
I think one flaw in the discussion, which was acknowledged, is that our present view of marijuana is skewed because of our current system of laws and our related lack of evidence. We have plenty of evidence about the positives and negatives of alcohol, but marijuana is still an illegal drug for much of the world, so not only is there not much acceptance, there's downright criminal punishment.
The idea of alcohol and societal cost is a little foreign to me, not because I can't see the societal cost, but because the vast majority of people I know don't contribute to that cost because they consume alcohol responsibly. If anything, alcohol has a positive societal cost for me, because I enjoy it, and because the money I spend on it is adding to the economy. That sounds like a justification, and it is, but it's true.
One economist's opinion was that, all things being equal, if we discovered marijuana and alcohol tomorrow, marijuana would be legalized and alcohol would be outlawed. I have trouble envisioning this, largely because of the delivery mechanisms. Alcohol is a liquid that goes straight from container to mouth; marijuana is typically smoked. I can't imagine smoking marijuana with dinner in place of wine, partly because smoke and food don't really mix, but also because of our society's stigma towards smoking in general. Mix it into a tea, and then we'd really have something. #science