The thing about hype is that it infects the fans, and in some cases whips them up into such a frenzy that they insist a game is amazing even when it's only fairly good at best. This is especially the case with established series that have dedicated fanboys (Halo, FF, anything by Nintendo) - they go blind and rabid with hype and the games get great reviews to appease them. Any criticism of the game is seen as pretentious or stupid, or simply not seeing the genius behind the game. And of course all the hype by the fans themselves makes others go out and buy it, and the game gets declared a best-seller, helping it establish its reputation even further.
Hype mostly goes bad with unestablished games. The recent Assassin's Creed for example, which was an okay game, but with a *lot* of flaws, and certainly didn't live up to the hype applied to it. There was no fanboyism to put down the intense disappointment and criticism after its release, and so it's been almost universally lambasted, scoring far lower in reviews than the developers were hoping.
Of course no hype for a game would mean it simply wouldn't sell well at all. I wouldn't call it an enemy of games. It can make new games seem like a disappointment, but it still gets them a lot of sales at the start. It's incredibly annoying, and almost comedic when the game turns out to be atrocious, but it's kinda necessary to get people aware of the games and make people want to buy them.