A few people have mentioned monocultures and it's worth pointing out that one of the biggest advantage of GM is not in really novel out there breeding but in simply doing the sort of selective breeding we did during the Green Revolution quickly and cheaply. Potentially then it can get us out of the monocultures that dominate agriculture currently, by letting us bring ancestral varieties of rice/wheat/etc, or currently marginal crops up to the same standards that we've raised the main wheat varieties and such like to.

It is my understanding however, that the incentives for this aren't there under the current economic set up