Most of the protesters I saw in Seattle were workers, unified in different kinds of groups. There were engineers, teachers, nurses, machinists, pilots and others. The reason for the protest is basically because the organization has no laws to protect the environment, labor or public health. Also, the WTO has power to strike down country's laws. Environmentalists see the WTO laws as contributing to air pollution, global warming, and deforestation.
Labors are basically worried about work conditions around the world.
For example, children are used as cheap labor in poor countries, which causes workers in rich countries lose their jobs. This organization is making rules, interpreting and enforcing the rules, with a secretive pro-corporate unaccountable to any public. The protesters showed their disagreement with the WTO in different ways. Others were wearing costumes. I saw turtles, girls without clothes, and some dressed as skeletons and clowns.