What's the big deal about GMO foods?

I must admit that I've been rather ignorant about GMO's for a long time now. I couldn't really understand what the big deal was about. After all we have been modifying animals and plants through selection for thousands of years. True, some of the results aren't great: cows' udders are known to cause them pain, persion cats can't eat properly because of their flat faces, etc.

I bought a book by Jane Goodall about food (I can't remember the title now) which described some of the problems of GMO food. Genetically modified foods have genes taken from other life forms and transformed genes which make them bigger, "better", and resistant to pesticides and herbicides. Some of the problems are

1) We are not sure what sort of negative effects food like this could have on humans.

2) Natural cross fertilisation results in genes being spread from genetically modified crops into other crops. This means that all crops whether you like it or not can and probably will become infected with GMO genes.

3) The spread of GMO crops is putting more and more of the land under monocultures with very little genetic diversity.

4) These genetically modified plants can spread everywhere and are resistant to herbicides and pesticides so won't be easy to remove.

5) GMO companies are gaining a monopoly over food and own the rights to certain genes.

Another question I've been wondering about is why is variety in genes important? The reason that I've discovered is that variety of genes means robustness. When a disease or change in environment starts affecting an area, variety in genes will allow some plants to survive and therefore prevent the extinction of the species. Genetic monocultures make humans vulnerable to food crisis.

Also GMO seed companies have patented their seeds, meaning that farmers are not allowed to grow these seeds without purchasing them, but with the natural cross fertilisation farmer's crops are being contaminated without their permission and then being sued by big GMO Seed companies!

It's just crazy and I certainly will be more wary of GMO food. With natural biodiversity already under great threat we certainly don't need to add further pressures. Also what are the ethical concerns of one or two companies owning the genes of certain life forms?