I read recently that the amount of tar from weed compared to cigarette isn't as much as people thought. It does contain more, but nowhere near the amounts originally reported.
Also, there has been no suggestions that weed contributes to lung cancer. "We hypothesized that there would be a positive association between marijuana use and lung cancer, and that the association would be more positive with heavier use. What we found instead was no association at all, and even a suggestion of some protective effect."
http://www.washingtonpost.com/wp-dyn/content/article/2006/05/25/AR2006052501729.html
I understand that everybody is different, especially how people react to things. But the point remains, taxing reasons aside, how can alcohol be legal and weed illegal? It's a gateway to other drugs? Isn't alcohol a gateway to smoking?