Well, for one, it's made by microsoft to further advance their plot of world domination. For two, the biggest hype about it is the Halo series, and FPS games on consoles suck (not to mention Halo is a fairly mediocre game). For three, consoles in general take away development time for better systems so we end up with crappy ports. For four, to put it simply, it has issues.
Actually, I don't really have anything against it, but I certainly wouldn't buy one myself.