A Console FPS done right!
The one thing that really stands out with Halo is the polish. The game feels like it's been carefully massaged until everything looks right and feels right. It almost feels like a first party Nintendo game in that respect.
The graphics are clean and at times can be impressive, particularly the way light shines on metallic materials. Overall though I wouldn't classify the game as a jaw-dropper graphically, but rather a well designed game with aesthetics that never detract from the game.
The music and sounds are among the best I've heard in a game. Perfectly orchestrated tunes drop in every now and then to give you a sense of epicness at exactly the right moments. The combat sounds all sound great and I especially like the voices of not only your allies but of your enemies. It gives the game a real organic feel.
The gameplay is fun and fast. The controls work great and the layout of all the buttons works well.
The story is above average for an FPS with an interesting plot twist halfway through the game. I wish the ending was better executed.
My biggest problem with the game is that two of the later levels are very bland and repetitive. Considering that this isn't the longest game, those two levels make up about 15% of the game.
I actually didn't try out the multiplayer, but considering that is what it is known for I'll assume that it is well executed. The game does have single-screen co-op which is awesome. I wish more games did that.
Overall, as you probably already know Halo is a very good game, not quite on par with the all-time greats in my opinion, but very good nonetheless.