Another reason that we may have trouble playing an imperial role is that, at the end of the day and for various reasons, Americans seem to WANT to be liked. There appears to be some sort of basic need for external validation and approval. This is, of course, a sweeping cultural generalization, but I've come to believe that there is some truth in it as well. One could argue that the British at the height of their Imperial power (or the French, Dutch, Spanish, Portuguese, and others) really didn't care if anyone liked them or not. There was a basic sense of cultural superiority at work that allowed them to take actions routinely that we would shy away from.

The older imperial powers didn't blend in with local cultures: they modified them to suit their needs. The Raj bore little relation to traditional Indian society at the end of the day. We're torn between wanting to preserve and be liked and wanting to remake things in our own image.

Sweeping generalizations to be sure, but as I said before I think there's enough truth in them to validate them on a loose conceptual level. The US really became imperial by default, not necessarily by design. And it shows.