Halo 3's single player campaign wasn't the only thing impressive about the game, it also has some pretty amazing multiplayer modes going for it.
I had initially blown off the need to play multiplayer during my trip to Bungie because I had played the game online with everyone else during the beta. But once they reminded me about their four player co-op mode and the Forge which allows you to edit multiplayer maps on the fly my interest returned.
Co-op will probably be the main reason I play the retail version of the game at my house once it officially launches. I've already beaten it on my own, so I don't really feel a burning need to do that again, but I would love to go through the game in Legendary mode co-op. While the game allows you to choose between playing as the Master Cheif, Arbiter and elites N'tho 'Sraom and Usze Taham it doesn't really change the experience as much as you would think it would. It isn't, for instance, like choosing different classes. I was surprised to find that the starting load-out for the Arbiter was a Plasma rifle and carbine, but no sword. I suppose that makes sense, because I know I would have spent the game running around with the limited charge sword shiving people, but it's still a bit annoying.
The co-op game also includes a "meta-game" which allows you to track your various scores, based on the number of enemies you defeated and how "stylishly" you dispatched them. This meta game means you can play competitively against other's in a co-op match.
From what I saw of the co-op mode it's a fun and meaningful addition to the franchise and one I suspect a lot of people will be using.
Forge is also pretty interesting. On it's surface it looks just like any other match, but by pressing a game you can instantly transform into a sentinel and then hover all of the map deleting weapons and items, and tinkering with where people spawn and such. While this isn't nearly as robust as a game with a true map editor, the ability to move weapons and items around does give creators a chance to stretch their creativity.