I work with lots of Americans and have done for years. For me, the main difference is cheesiness, cynicism and the degree to which things are taken seriously.
Americans take themselves and everything around them so seriously. There's no culture of 'fuck it though, let's just go to the pub'. They will tell you they love and respect you and they actually mean it in a non-piss-taking way.
Comedy is totally different, American comedy is based on expecting what will come next, British on the unexpected twist.
Also, use of the word 'cunt'. When Americans use it, it sounds really awful and crude, when we use it, it sounds natural and normal.
They're also really really open. I met an American woman the other week who started telling me about her marriage problems and infertility within 5 minutes of meeting me. I have best friends I know less about than I now know about her!
I'm mainly talking about white Americans though - those from other backgrounds/cultures tend to have a different take/more cynical.