The press has recently attended preview sessions at Bungie HQ.  They've gotten to play parts of the campaign and magazines have been given renderings and screenshots to publish.  HALO 3 looks mature, spectacular, and deeper than HALO 2.  See what I mean:

Scans of Famitsu H3 images from Neogaf.com
Scans of Joypad H3 images at JeuxFrance.com
Summary of Interview with Jamie Griesemer from MSN.ca:

The Scarab conflict is part of The Storm, the fourth mission of the single-player campaign component of "Halo 3," and just one portion of the mission, has more enemies and allies than the entire first mission of "Halo 1."

The size really ramps up as you go through."But in 'Halo 3,' The Scarab is an AI (artificial intelligence). He'll chase you around, he'll acquire targets. If he can't find you, he'll search for you. He's the biggest character we've every done, essentially. And there are dozens of ways to kill him."

"All equipment in multiplayer shows up in single-player and the AI will use it on you," said Griesemer."The main way you're going to get equipment in the single-player game is by killing guys before they can use their own (on you)."

On the multiplayer front, a nifty feature called The Forge allows the gamer to redesign maps by changing the placement of weapons and vehicles, as well as changing the rules of engagement.

The game becomes more complicated via hidden skulls. Scattered through the game, these change the rules - for example, one skull calls for both players to die and be sent back to their last checkpoint if just one player is killed.

The ultimate task will be to finish every mission on legendary difficulty, with all skulls turned on, in a certain time limit with style.

It's looking like HALO 3 will not disappoint on the campaign front.  We are going to get more vehicles, more enemies, better vehicles and better enemies.  We are going to get rich environments and complex engagements that are sure to add dozens, if not hundreds, of hours of replay value to the campaign.