I think the problem with many people is that they interpret "I'm really excited about this game!" as "This game will change gaming forever and sell over 10 million!"
If you scroll through my posting history, you'll see that I've probably hyped this game more than anybody else on this forum, yet the entire time all I've said was that this game plays great, looks really great, and has an addicting multiplayer experience. It's just fun. It won't reinvent the wheel. It won't become the best game ever. It won't do anything like that. But it'll be an awesome game.
And then people come in, bitching about overhype and whatnot, saying people will only be disappointed in the end. Yeah, you might be disappointed, only because you interpreted the excitement in an unrealistic way, most likely only so you could later use it as ammo against fans of a console you don't particularly like.