The reviewers are mostly correct when they say that there's very little new in "Halo 3". It's a very well-executed game, the story is competent enough, and the multiplayer component is a blast. But the controls are roughly the same as in "Halo 2" (the button scheme was rearranged slightly to incorporate the use of special equipment like deployable cover and tripmines), and the gameplay is pretty much unchanged from the standard run-and-gun of the first two installments.
But one place where the game really innovates is in the Theater feature. The Theater allows you to record your exploits in the "Halo 3" campaign or in multiplayer, play them back, screenshoot them, and then collect them from Bungie.com through your "Halo 3" Service Record. So, when you drive your ATV up onto a crane arm and jump on the back of a gigantic walking robot, you can record that moment and play it back again and again. This is an especially cool move by "Halo 3" developer Bungie because of the "Halo" franchise's history with the machinima community.
If you haven't heard of machinima, click through that Wikipedia link -- it'll give you some good background. One of the first breakout hits of the machinima genre was "Red vs. Blue," a comedy short series filmed using the Halo engine. The series went five seasons, and was enormously popular, attracting a million viewers per episode.
But the process of creating machinima isn't very easy, especially for the less technically inclined. By offering the Theater feature, Bungie has brought the possibility of these creations to a broader audience--something that few video game developers have ever bothered with. Although it's unlikely that any high culture will come from "Halo 3"'s Theater (the vast majority of videos probably feature people killing their friends in remarkably humiliating ways) bringing an element of content creation to an otherwise straightforward genre is definitely an innovative move.