I also have had negative reviews not published and have felt, therefore, the same as the OP that reviews may be heavily vetted. Having said that, I did post one which had a slight anti-product reference to it and it was published (panasonic hdd/blu ray), so I really can't comprehend what the policy for publishing reviews is. Perhaps it depends on the product.
The next day I got an email stating that my review had been looked at and posted. Maybe I took the rejection personally but unless someone used profane language, used the name of another product or shop or something similar, I cannot see a valid reason for not posting all reviews submitted. That's the only way buyers will have confidence that reviews are legitimate.