At a recent conference experts and scholars discussed this issue:
Americans equate freedom with the ability to practice religion openly, Europeans believe freedom is gained by confining religion to the private sphere. Craig Kennedy, president of the German Marshall Fund, an American institution fostering U.S.-European relations, said most Europeans are confounded by the eclectic nature of American religion. "In the U.S., the separation of church and state gives us the insurance that allows us to have a freewheeling, dynamic religious system that seems quite chaotic to Europeans," he said.
It may be that much of the anti-American feeling that has swept Europe may be attributable to the growing realization of just how religious many Americans tend to be, a sharp contrast with the more reserved and private religiosity of Europeans. It's not so much that Europeans are less religious (although some countries are), but that the modes of religious expression and feeling are very different - despite the fact that both share a common tradition of Western Christianity.