David Denby makes this claim, first, I think, in a New Yorker blurb published after No Country for Old Men was released (I read it in his book Do the Movies Have a Future?, which I would recommend, but not as much as David Thomson's Biographical Dictionary of Film, which I highly, highly esteem). He says, this is the first film in which the Coens tell the straight story, so to speak, whereas all of the previous films were comedies, the punchline of which was, look at these stupid pathetic humans.
I'm shocked, actually. This seems right, in some respects. But I do not think it is.