I actually love to study genetics and plants and personally don't have an issue with the idea of genetic modifications. My only issue with it is what in the past we've been modifying towards and mostly the impact that these modifications have had on local ecology... (Such as the herbicide use as mentioned, the ability for these crops to become superweeds if they escape the crops which is actually a major concern with drought-resistant GMOs and the like, the effect the GMOs that produce inherent pesticides have on the insects that would naturally eat them, etc.) As for tree screens and distance standards etc. between crops and wildlife; its debatable how adequate the current standards are... However, I've read several studies that have shown that this issue might not be as of much concern as I personally give it... who funded those studies, I am not sure...
The change in the plant itself could easily be more nutritious, if we were to choose to do so. My only issue with that is in the past we've chosen big, pretty, shiney fruits/veg rather than more nutritious. Were GMO to become accepted within years I'm absolutely sure we'd see "Tomatoes, now with Omega-3!" and "High Fibre Peaches!" and "Broccoli with Vitamin B12!" and so forth--something I personally would definately consider adding to my diet.
Wednesday: the genes themselves are harvested from whatever it is has the gene we want; if we want frost resistance (as was given to tomatoes that sparked the fish allergy controversy) then we get it from atlantic fish. If we want disease resistance we likely get it from a strain of bacteria. If we want something plant B has, we get it from plant B.
There are several ways to produce GMO plants with this DNA and most (if not all..) of these ways include bacteria, as there exist several sorts that specialize solely in corruping plant DNA and so you can see how easy it would be to use this bacteria to add whatever DNA you want to your plant, simply by putting your DNA in the bacteria (which is relatively simple to do!)