I believe I've finally worked out my personal theory.
Here it is :
In person, Americans are wonderful people. Funny, intelligent, generous and always up for a chat. This was brought home last night on the train, when I was sat (building some lego, oddly enough), and an American woman started up conversation. She was great to talk to. I've never met an American (in person) I didn't like.
However, put them on TV or on the internet and it's more of a mixed bag. Whilst some will be just as lovely (everyone here, for example!) some are predisposed to be arseholes. Which describes the internet in general, not just Americans, really.