My impression is that No Country for Old Men will be significantly darker and more serious than most Coen’s movies. The poster seems to confirm that impression.
It’s a simple poster really. A man holding a shotgun running through some desolated landscape, with Javier Bardem’s menacing eyes looming behind him. But it does a good job of setting up the movie’s theme and setting. It also gives us a good idea of the atmosphere we can expect, mostly though the use of very dark colors.
Not an amazing poster, but certainly a decent one.