There is nothing inherently wrong or dangerous about GMO as a technology. It is more the way in which it can be (mis)used that can cause problems. For example, one of the primary uses of the technology by corporate america is to create food crops that are resistant to herbicides (Roundup ready). That way, farmers can apply tons of herbicide to control weeds without damaging the crops. Then these chemicals make their way into streams and rivers causing environmental damage.
Another potentially problematic use of GMO is altering food crops so that they produce their own insecticide. From an environmental perspective, this is potentially a good thing. Now the farmer doesn't have to apply any insecticide to the crop. But, of course, great care must be taken to ensure that it is safe for consumption.
Furthermore, these types of GMO make it easier to grow food as mono-culture which are environmentally damaging. If you have ever been to the american midwest, you have no doubt seen the vast expanses of nothing but corn and soybean fields. This type of agriculture causes extensive breakdown of soil structure resulting in erosion and loss of vital top soil. Nutrients are extensively depleted making manure and synthetic fertilizer application essential. Again, all of the runoff makes its way into streams, rivers, and eventually the ocean. Just look at the increasing size of the anoxic zone in the Gulf of Mexico.
Even conventional plant breeding can lead to food crops that have negative health consequences. Take wheat for example. Modern day wheat is very different from wheat which was cultivated hundreds of years ago. It has been selectively bred for shorter stalks (better wind resistance) and bigger heads. As a consequence of this breeding, the chemical makeup of the plant has been altered as to make it harmful for many people (gluten sensitivity/ celiac).
Of course, its not all bad. GMO gave us glow in the dark fish and cats! Seriously though, there is a huge potential for good things to come of the technology. And, of course, huge potential for bad things to happen. It is a very complex issue.