I remember reading a blog post somewhere describing the problem with modern recommendation engines being their reactive nature. Say you purchase a cell phone on Amazon -- they'll start showing you other cell phones, cell phone accessories, more electronics, etc. The problem is, you just bought a phone -- you don't need 5 more, and you max need 1 or 2 cases. So the recommendations are garbage, since the engine has no way of knowing what your current mindset is and what you actually might be interested in.

In hindsight, that's obvious, but it struck me at the time. I don't ever want to see my thoughts get read by a computer (personal choice), but until such technology exists, recommendation engines will continue to suck.