This game was my introduction to Xbox (as I'm sure it was for many others). I believe this was the first FPS that I played since Goldeneye and what a staggering leap forward it was! Granted, dual analog sticks were a big part of that. But the world that Bungie Studios built was so intriguing and inviting. And the story was actually quite impressive, particularly the twist partway through where you learn that the Halo ring's true purpose was to wipe out all sentient life in order to remove all source of food for the Flood.
And it's the Flood that really elevated the game - it was a more terrifying enemy with how they utterly swarmed you. Plus, that's when you got the shotgun which is the best gun in the game.

But what really pushed this game into the stratosphere was the multiplayer! Single player was fun for a time, but multiplayer is what made me, and my family, and my friends want to return over and over again to this game.

However, as groundbreaking as this game was (and without it, it's quite possible that Xbox would not have survived), it doesn't hold up all that well today. The warthog is atrocious to control (which is thankfully fixed in the sequels), the enemy variety is limited (only 8, half of which aren't seen until the last half of the game), and most of the guns (particularly the covenant weapons) are underwhelming (particularly when compared to later entries). It was also easy to get lost at times (particularly the original version and its poor lighting).

If I hadn't played this when it first came out, I'd probably only be able to give it 3 stars. But due to my nostalgia for it and my appreciation of its importance to gaming, I think 4 stars is warranted. It's a solid game, but the sequels is where the franchise truly shines!