Also, to be completly fair you really should compair graphics to games with the same scale. i.e. It is much easier to have better looking creatures if there is only going to be 2-10 fighting at once. (Such as Bioshock, Quake, Gears, etc.)
In Halo, the scale of conflict is much greater, with somtimes 20+ opponents on the battlefield with aircraft, vehicles, etc.