Food experts needed

I've been seeing these billboards for McDonald's, advertising their new chicken sandwiches. I don't go to McDonald's a lot, but those sandwiches just looked so good that I couldn't help it.
I had the Chicken Club, and I have to say that I was disappointed. The old chicken sandwich was just a processed meat patty. I like the processed meat better. It's always the same consistency and always tender and juicy.
So this gets me thinking. What's wrong with processed food? I mean, it's got such a bad rep, but why? I have no idea why it's considered bad for me. For that matter, I don't understand what all the hype (good or bad) is behind organic foods, preservatives, or even fresh foods. What kind of motivation do I have to eat "healthy" when I don't know why these things are considered healthy at all?
I could draw some kind of analogy between this and other areas of my life, but I think I'll keep that to myself.