A few other people already said this, but Halo is very good when it comes to using colors. Its very vibrant and energizing, and feels much more varied than other games. Also, you fight more than just one character model, which adds variety to the action. The reason that I never finished Battlefield: Bad Company 2 or Red Dead Redemption is because the enemies behaved exactly the same, there was no sense of strategy or accomplishment for getting though a fight, which is a shame for RDR due to all the hard work the narrative gave to try and draw me in. Thinking about it now, what would Mass Effect be if you took out biotic powers, or its sci-fi theme, would I have ever loved that and finished it so many times? When I heard that Mass Effect 3 was going to make enemies more dedicated and varied with more specialized roles, I was very excited.
The versatility of the custom game system has bred hundreds of amazingly creative and entertaining game types, which combined with the Forge map editor creates insane things that you can't get anywhere else (except maybe Little Big Planet, but I don't know, no PS3). For example, did you know that someone actually made a working game of Skeeball, and Bungie was so impressed that they made a new gametype to streamline the game and used the creators map and put it in Matchmaking? When you get into it, Halo is much more unique than some would suggest. CoD just doesn't have that spark, it feels rigid and inflexible, and there's too much like it.
And that one level in South America was just bullshit, especially on veteran.
