I think this is the tragedy of the commons in action. In this the case, the "common" is our collective willingness to post helpful information on websites for other customers. From my point of view, I'm adding value to the merchant and getting nothing in return. I used to get the fuzzy warm feeling of contributing to the collective good of humanity, but now that it's so commoditized I'm not willing to do it anymore. There are two solutions that I know of; the socialist one and the capitalist one.

The socialist solution is some kind of collectively-funded watchdog organization that provides useful review information. The capitalist solution is an organization that straight up charges for reviews, ie, Consumer Reports.

I'm sure someone's working on their startup about it now.