McGrath and John make good points about strings left open even when you're fingering way up the neck... John Denver used a kind of 'acoustic power chord' to good effect this way. And I've always been fascinated by something that Leonard Cohen did on his first album - he played a C shape, without a barre, at (I think) the third or fourth, followed by an Am shape at (I think) the fourth. All other strings left open... really good effect.

- Ian B