Yeah, why are people against NAFTA? I truly don't understand that.
The most solid ground to attack it is the Mexican farmer suicides. Which is a direct responsibility of the protectionism the U.S. encourages with its outrageous corn farm subsidies. (And yes, I know the EU is just as much a villain in this respect)
Luckily, the TPP seems to undercut the Japanese corporate agribusiness a bit, which is very good because they are a terrible racket and quite surprising seeing as the LDP are in power at the moment.