Decent Story, awesome gameplay. But not quite as groundbreaking as Halo 1.
For one, level size. Levels are a lot longer in this game, which makes for a more dynamic, more satisfying approach. But in each part of the level, you are always in tight spaces. You aren't in wide open spaces like in halo 1, and you are always in narrow hallways, passages, or just running around with too many obstacles in the way. Then, theres the fun of the game itself. Halo 2's single player is not nearly as fun as Halo 1. This is because of how the game is way too one sided on having vehicles incorporated into the game, so whenever there ARE big enviromental spaces, you are required to be in a vehicle because not much enemies are on the environment. Also, the rumble effects have been toned down A LOT. In the first game, you had a lot of rumbling, followed up be a lot of hard loud sounds coming from your gun. None of this is true of Halo 2. But, as a stand alone game, not being told from as a comparison to another game, Halo 2 is very good. And it's Multiplayer, well, see for yourself, it'll blow you away.