And the problem with those arguments is that, while they do sound good, with a bit more context and information you realize they are actually vary poor anti-GMO talking pieces. If you did those exact same things with conventional breeding, no one would care.
they make plants that produce chemicals to kill pests, with possibly unknown health effects
All plants do this. Plants cannot fight insects, so they produce insecticides. Caffeine in coffee is actually one of them; why do you think the plant produces it right in its seed, its offspring? Not so something can eat it, although by a twist of fate that wound up being what we consume it for. Adding an additional insecticide is not, in and of itself, concerning, and in the case of GMOs, the one added comes from Bacillus thuringiensis, which has been sprayed on organic crops for years with no ill effects. We know how it works and its mode of action. It does not affect mammals. I previously stated that no one would care about this if GMOs were not involved; how do you think pest resistance is bred conventionally? There is work breeding high maysin (a natural pesticide in corn) lines of corn, and no one cares. That's because the arguments against GMOs always follow the conclusion, not the other way around (that's why even things like Golden Rice and Arctic apples have arguments against them; don't be surprised that these have opposition arguments cooked up too).
they make plants that are resistant to herbicides, which promotes the use of these herbicides, which promotes the development of superweeds
They make plants resistant to certain herbicides, specifically glyphosate and glufosinate. This allows a shift in weed management practices away from harsher herbicide, and soil damaging energy intensive tillage, toward more benign, selected herbicides. I'd rather farmers spray glyphosate than atrazine or use tillage. And again, no one complains about Clearfield wheat, a conventionally bred herbicide resistant line, and no one complained about the herbicide resistant weeds that have been appearing since the 70's (and please, they are not 'superweeds' any more than the GMOs themselves are Supercrops). Furthermore, if the herbicide resistant GMOs offered no benefit, why would weeds resisting their herbicides be such a bad thing? The anti-GMO movement is trying to have its cake and eat it too, saying there are no benefits to herbicide resistant crops (there are) AND the herbicide resistant weeds are threatening to take away their benefits. Unfortunately, it seems like no one calls them out on this logical inconsistency.
they patent everything and engage in licensing schemes that are really harmful to small farmers
Of course they patent everything. Those of us who work in plant improvement have a right to make a living. Lots of non-GMO crops have been patented since the plant patent acts passed in the 30's and 70's, and rightfully so. Do you work for free? I'll bet not. So why should plant breeders and genetic engineered? If you don't want to use those patented crops, don't. Ever had a pluot? Did you know they are patented? They took decades to develop, is it any wonder the breeders would like to maybe not go bankrupt and continue to produce something valued by society? Furthermore, Monsanto's first GMO soybean goes off patent this year and will be able to be freely planted in to 2015 season. Isn't that how it is supposed to work, develop something, make money, it goes to the public domain? I fail to see the problem. As for it hurting small farmers, that is false, they use GMO crops too. They don't have to, but they also get benefits from it. Why would new technology hurt small businesses?