This is also a problem when looking at books on the Kindle. Many books are good, but their reviews are destroyed by a terrible Kindle conversion. See the version of Surface Detail by Iain M. Banks, for example: http://www.amazon.co.uk/Surface-Detail/dp/B00462RVHI/ref=sr_...
I agree, but in the case of bad Kindle conversions, the only thing Amazon can fairly be faulted for is requiring users to accompany their reviews with ratings. It's up to the publisher to perform the conversion.
Whenever I have a complaint about a Kindle conversion (twice in two years), I try to rate the book based on the author's work -- the text itself -- but make the review title something like "Do not buy for Kindle", but I can see why reviewers might choose to do otherwise: it is, after all, the publisher's product and the publisher's fault for producing a sloppy "printing".
