Finish the fight. That's what it says on the box. And oh what a hell of a finale it is.
The campaign. Filled with epic set-pieces and incredible gunplay. The story picks up moments where it left off with Halo 2, the Master Chief, anonymous badass, landing on Earth. I won't say more, because you want to see this for yourself. The gameplay here is perfect. I really don't know what else to write. It's just great. Every moment, (except that damned 8th level) is absolutely brilliant, always filled with action. The most downtime in the games is probably about 1 minute. The rest, you are giving those Brutes what they deserve. The grunts are hilarious by the way. I won't quote any lines, because I don't want to spoil it for you, but they are my favourite enemies of any game.
The graphics. This game is absolutely gorgeous. The textures are incredible. You can walk up to them with the sniper rifle and zoom in as far as possible (10X) and they hardly look pixelated at all! The lighting just makes you want to explode, it's so beautiful. The only minor downsides are that the marines don't look as great as the rest of the game, and the jaggies are very obvious and in your face.
Forge and Theater. 2 of the best parts of this game. With Forge, you can create all kinds of crazy stuff, like gigantic robots, catapults or even rocket ships. Getting bored? Make a castle! Want to see ragdoll physics in action? Put a whole bunch of explosives in a small room and watch the bodies fly. And if something particularly cool does happen, you can save the replay in theater mode. Here you can play, pause, rewind, fast forward, slow-mo, take pictures and save clips! The only downside is that you can't save clips from the campaign, only entire levels.
The Multiplayer. Oh god the Multiplayer. There are so many maps (DLC ones included) and game modes that you won't know what to play first! There's the classic Slayer, CTF, Territories, Oddball, and King of the Hill modes, along with Grifball, Infection, Sumoes, Tug of War, and many many many more with variations of each. All the guns are so balanced (except the slightly too powerful BR) that you won't get frustrated unless you suck. Here there is a big skill gap, so prepare to get your ass wooped for you first games. When you get the hang of it though, it feels incredible to pull of triple kills and Killing Frenzies.
Feel like mucking around with some buddies? That's cool! Every single mode has a private mode, where you and a select group of friends can chill and play baseball. Yes, baseball. 1 person with a rocket launcher and one with a gravity hammer. You know what to do.
Pros
Campaign
Multiplayer
Graphics
Balanced
Theater
Forge
Cons
Legendary ending too much of a tease
8th level