There won't be a particular incident that will make people realise that hotel reviews done by Joe Public are not worth believing.
It will be a combination of revelations, individual experiences and Twitter and social networks that will blow away the myth that reviews on hotel review sites are truthful.
One of the problems review sites have is the temptation on the part of hotel owners to submit fake reviews. For instance if a PR company approaches a hotel and says they can get the hotel favorable reviews, thus boosting it's ranking. It can be hard for a hotel to resist this.
The trouble for review sites is that the number of reviews being placed on review sites is increasing yet they have no real way of checking if a review is true.
Various articles and questions have been raised about this.
Arthur Frommer from Frommer travel guides has voiced concerns about this for ages, here is one of his articles about it http://bit.ly/cTt9ZL . Then there's Christopher Elliott, a champion of travelers rights here what he posted http://bit.ly/wWzuS .
These are respected people within the travel industry but soon individuals will start to realise this. And while individuals in the past, did not have a way of letting anyone know what had happened, now they'll start to post their complaints on Twitter and their social network.
That in itself still wouldn't kill of these type of reviews. But with Google starting to include reviews from Twitter and social networks in search results. You're going to be left with so many conflicting reviews about a hotel you're not going to know who to believe.
Once you get to that situation you're going to want to find a trusted source and that won't be a pure review site. Who knows the expert knowledge of the traditional travel agent may become the source people turn to.