Michael, right on. This is at the heart of OpenReviews, structured blogging and micro-formats; you clearly see the potential (and have articulated it better than most)!
While I certainly can’t speak to Amazon, I do know that a fair bit smaller site is working on implementing exactly what you’ve described for their members (and by extension of the implementation, the web at large). I can’t say more than that right now for the sake of a couple of personal and professional relationships, but I hold high hopes for these folks to get it right, and shine a light down the path for the rest of us.
Michael, do you think it could work for a health site too?