A recommendation engine should be a mixture of at least these contexts:
1. similarity to previous choices
2. overall popularity (or some measure of quality): recommends most popular items
3. novelty (newness): recommends newest items
The recommendation engines I've encountered seem to be stuck on #1, so they just keep feeding you more of the stuff you've chosen already. They are seemingly unable to learn concepts like "user has eclectic taste with a preference for novelty" or "user has eclectic taste with a preference for quality."
It could be that this has been done and I just haven't noticed it. I don't buy much online lately.