It's not just slashdot, of course, but it's especially common there because slashdot readers tend to think that they're clever.
The basic problem is thus:
"I am a smart and sensible person. Therefore, all sensible people think what I think."
In terms of video games, this commonly plays out in one of two ways:
"I don't like this game. Therefore, no sensible person would like this game."
Commonly, if a Sensible Slashdot Reader encounters a game they don't like, they insult the intelligence of anyone who does actually like it, or outright assume that absolutely no one likes it. If, for instance, an indie making a complex business sim posts about their less-than-rockstar sales results, a horde of SSRs will post "Your game isn't selling because no one would pay that price for a game which is ugly/not 3d/on a PC/lacking tits/lacking guns/etc." If a company mentions that they're releasing a game not targeted at the typical slashdot market, the SSRs will laugh and point and proclaim that the company is doomed and deserves to die, because no one would buy games like that. Faced with evidence that people actually do buy those games, they will retreat to the earlier position of "Those people are brain damaged."
The flipside also applies:
"I like this game. Therefore, no sensible person could dislike this game."
Faced with the slightest hint of criticism against a game that they LIKE, the SSR will immediately assume that something is outright wrong with the person posting criticism. They tend to then invent a possible (but very stupid, since after all, the critic MUST be stupid) motive for why the critic doesn't like the game. This motive may be an extremely distorted version of the actual criticism, or it may have nothing to do with the actual criticism whatsoever. Then loudly proclaim what idiots the critics are because they believe $FabricatedReason.