As with every other thing that is produced as part of a product/payment process, genetically modified crops can give a big blow to farmers all around the world in that the product that they already have set to grown and cultivate might be in danger of not being consumed. This will impact the economy of many cities and countries, especially those who depend on the export of certain crops. For example, if potatoes were to be genetically manufactured in California, Idaho would lose its main produce and their economy (and job opportunities) will go down the drain. This is the ethical issue to deal with.
In a moral level, to which point can scientists reproduce what is already naturally available? Many feel that this could result in a basic disrespect of mother nature, not to mention that if the crops lose any of their original nutrient value the entire world could become equally affected.
Finally, health wise, a study in Europe pointed out that some GM crops affected bees in certain ways, which will affect pollination and the natural progression of things in the food chain. Speaking of the food chain, humans may develop new allergies to the genetically modified foods since they will lose or change a lot of their original genetic make up. Not enough tests nor exposure to these foods can correlate to any possible future negative reactions to them, so we need to keep a close look at trends in research in the future.