70% good, 30% needed impovement.
The opening cimematic really shows how the Chief and Arbiter are bickering, yet they still work together in the next few levels. Even though Johnson is a main character, I felt like rescuing him being the main focus of Level 1 was a waste and Bungie could have used it for a better purpose, but it was more like an introduction to Halo 3 for newbies, so I guess that's okay. Though the first levels of Halo 1 and 2 were more key to the overall storyline of the trilogy.
Crow's Nest, a display of the Master Chief's awesome skills. Also, this level does a great job of introducing the Drones to newbies, because you don't really expect them when returning from the hangar. It also gives fans something to talk about because in the end the Chief destroys the base, eliminating hundreds of Covenant.
Levels 3, 4 and 5 happen one after another, this is how levels should be. In level 3, you journey to Voi to assist in a great battle in level 4. The Tsavo Highway really seems like a broken path, road signs being extremely realistic. And when you finally arrive in Voi, a huge Scarab Tank appears out of nowhere, and you thank your lucky stars for the rocket launchers.
Floodgate was a very poor introduction of the flood in Halo 3, because they just came through without any warning, making you wonder why, when and how. Though it was very clever to transform level 4 to make it look like the flood had landed, and to make the player go through it backwards. Cortana's message also played a key role in the story, making all of that flood-killing worthwhile.
The Ark makes a new story arc in Halo 3, one for the Halo universe history books. Making a landing pad, killing a Scarab and finding the Cartographer made this level really action packed. Though, finding the Cartographer was kind of a bad move in the story, because they had already taken many things from Halo CE, eg. The Assault Rifle, non-homing Rocket Launchers.
Destruction of the 3 towers in "The Covenant" was a very important part of the story, and it was a really big decision for Commander Miranda Keyes to make when she ignored the flood and went straight for the Heart of the Covenant: The Prophet of Truth. The two Scarabs was a great decision, Bungie made it so that the player would face many Scarabs in the 10 levels, but not too many, for that would make the game kind of boring. Also, the Flood joining up with the Chief was a good, new addition to the story.
Cortana: maybe the level that had most to do with the story, the AI that had been missing since Halo 2 and had been popping up on our TVs finally returns. This level shows how technology isn't always better, as you witness Gravemind corrupt Cortana's data. The difficult to navigate through area made this level more interesting, but kind of confusing, because Flood are trying to murder you while you clumsily attempt to find your way to Cortana's "cell".
The final level. Again, kind of ripping off of Halo CE's 2 Betrayals and The Maw. Also, it's kind of strange how Arbiter survived and the Chief didn't. This creates a big cliffhanger and a possibility of a Halo 4, but I think they should have just closed up the story there and make a Halo with another story maybe, instead of a possibility of another sequel.
Now, Multiplayer. The maps are much smaller than Halo CE maps, maybe because we move faster or something, I don't know. Game types like CTF are still fun, but online when the maps are packed, the players could use a little more room.
Also, the heights of the maps are a little annoying. You're flying a banshee in Isolation when suddenly, you blow up. If the height of Isolation is so low, why even make the option of a Banshee there? And maps that would need Banshees like Narrows, don't have that option. Forge is also very tough to do, objects bounce around and don't fit through doors very well.
Customization of Armor is a very neat option, but the fact that you need Xbox Live to get wide varieties of armor makes it kind of annoying for offline players.
That is why I feel Halo 3 deserves a 9.0, nothing less, nothing more.