Suppose that marijuana was legal worldwide. What effects would this have on crime, drug trafficking, and society as a whole?
Basically what Nephi said.
Most of the reasons it was banned IOTL was because racism. If it was more generally popular with white people prior to it being made illegal, there would be less impetus to ban it, or at least to keep it banned - see alcohol prohibition.
If marijuana was never made illegal, then the legal version would probably not be as strong as the illegal version.
By making it illegal, it becomes more cost effective to the producers to have a more highly concentrated form. The same thing happened during alcohol prohibition.
If marijuana was legal, it would be subject to stricter quality control, and age restrictions. Besides, alcohol is much more dangerous to people than cannabis is, and is still legal, mostly because it is also consumed by white people, including legislators and their donors.
Of course, there would still be a black market in cheaper, untaxed marijuana, just as there is for tobacco and alcohol products.