I'll echo /u/Mouse_genome's comment, with a little more.

The deal with GMO technology is that it concentrates power in individual people/organizations/corporations. You can do amazing things with genetic modifications. The development of "golden rice" is pretty awesome since it could potentially eradicate vitamine A deficiency -- a major problem in some parts of the world. The Arctic Apple and Flavr Savr tomato are pretty cool if more frivolous.

But the amoral character of large corporations (the major organizations that have been marketing genetic modification), together with their drive to create localized profits, means that the power of GMO development is not always 100% unmitigated good. The development of Bt corn sounds like a great idea, except that it involves introducing a toxic pesticide directly into food crops -- which means that it can't be washed off the crops and enters the food stream. (Also, Bt corn has the same problems as other pesticides in reducing insect biodiversity yada yada). The development of "Roundup-ready" corn has led to the widespread use of glyphosate ("Roundup"), which is certainly having effects on the plant biome and spillover effects on insects (e.g. there isn't nearly as much milkweed as there used to be since farmers spray universal defoliant everywhere, this has led to a crash in the Monarch butterfly population). Glyphosate also has the usual troubled and ambiguous research history as far as human toxicity, and could be a problem in the human food stream.

There is another class of difficulties with GMO foods, which is that they change the economic system of food production, diverting huge profits to the corporations. Basically, you can't breed the seed corn from most GMO products, and if you do somehow manage to do so you get sued. Monsanto's profits have been, erm, large throughout this millennium so far -- mostly because their products are so good (on a year-to-year crop return basis) that they have come to dominate the U.S. food supply and make inroads into the world food supply. In effect, they've managed to convert farms (even large, corporate ones) from standalone food sources to sharecropping outfits beholden to Monsanto for their continued existence.

Given the track record of players like Monsanto, there is little reason to trust them with the public welfare -- but the genetic engineering "lever" gives them a Hell of a lot of political power, in its distilled form of money. That is far more concerning than the actual fact of genetic manipulation -- which should not be concerning at all.