Halo 3 has not changed or innovated a single area. It plays like Halo, looks like Halo 2 and sounds like ****. The soundtrack is good, I must say, but the weapons all sound rubbish, the campaign is short, the story is a weak piece of trash and the characters turned out to be a bunch of people that I really couldn't give two ****s about by the end of the campaign.
It's not as bad a game as Far Cry 2, I reserve a special kind of hatred for that, but what I will say is that Halo 3 is vastly overrated. I'll probably get flamed for this but whatever.