Word of mouth is one of the most powerful sales driving force there is, if not the No. 1.
Reason, it's because not all person are on the Internet of certain circles that allow them to get the sccop of the latest games, if they're good or not, what's the is about etc. Specially parents that want to buy games for their children are not that informed and don't have much time to get the news and propper information.
This is where word of mouth comes in.
It's in human nature to listen to people you usually trust about certain topics, that includes products: What is the best soap, detergent, ice cream brand, TV, car, etc. Games fall into this type as well, what is the best game to suit my needs, or my chindren's, which game is fun, which one is boring. Even before the Internet expansion, before gaming magazines, Word of mouth was present as one of the sales drive force.
What you call "negative", it can be anything. Like a warning to not buy a game that is really wont worthy, or to just troll and give misinformation so a good game is not bought. But what drives the course of creativity is not the word of mouth but the actual sales. I know evidence of good word of mouth helping up wuth the sales. Specially with the so called "evergreen" games. However, on the negative side I don't see much, because people would rather not recommend at all instead of just giving a warning.
