I think it's worth keeping in mind the fact that supporting marijuana legalization is not necessarily the same thing as supporting Prop 19. While acknowledging the dangers of legalizing pot - from the potential increase in usage rates to the unpleasant likelihood of a Big Marijuana lobby - I completely support legalization. There's simply no reason for pot to treated much more seriously than alcohol.
That said, Prop 19 is far from perfect. Besides being poorly written, here are some other problems:
1) We have no clue how much tax revenue it will bring in.
First of all, Prop 19 is remarkably vague about the taxing scheme. The nature of the taxation structure will have a major effect on total government revenue. Will counties compete in a race to the bottom on taxation rates in order to attract marijuana grow operations? What will the tax rates even be? Second, tax revenue estimates have been all over the place. Proponents suggest California could reap up to several billion dollars per annum, while the California Board of Equalizations predicted annual revenue of $1.4 billion. However, the RAND Corporation Drug Policy Research Center's report on this issue suggests the number could be far larger - or smaller - than that. It's simply impossible to predict. Third, we have little understanding of a number of related issues - like the kind of tax evasion we can expect, the effect of a legal California market on the rest of the country, usage rates of legal pot, and the popularity of the personal growth allowances in the proposition.
2) How will the federal government react?
California, and the other medical marijuana states, are already operating in a sort of legal gray zone. The Bush administration raided pot dispensaries and arrested many individuals involved in California's legal pot trade. Obama's administration has basically decided not to do this. Legally speaking, both administrations seem to be on solid ground. It's extremely unclear whether or not California could get away with legalization, or what would happen. Maybe Obama would choose to crack down on west coast pot, perhaps to look tough on a law-and-order issue? Maybe Obama wouldn't do anything, but another future administration would? Some even argue that legalization would be worth it, even/especially if the federal government cracks down on it, because it would open up the topic for discussion.
3) How will the rest of the country react?
California can easily grow enough marijuana to supply the entire United States. One possible scenario has underground market marijuana distributors acquiring up to 100% of their product in California - perhaps even legally, thanks in part to the now-lower prices - and simply driving it to the rest of the states (well, the other lower 47 anyhow). This could seriously piss off the citizens and politicians in these other states, which could then result in interesting political repercussions for California.
I'm sure there are more issues that I can't think of right now.
Anyone seriously interested in this topic should read the RAND report
here.
That's a long report, so here are some much shorter papers on some of these issues:
This RAND working paper discusses the ability of California to supply the rest of the US with pot.
This RAND working paper discusses excise taxation and tax evasion, as it applies to marijuana legalization.