You may never sleep after you discover halo 3 multiplayer!
Multiplayer: You will never find someone not playing. It may be annoying if you're matched up with people far superior to you, as it used to be with me. It may also be annoying if there are gamers far inferior to you, as is my case now, where I only go for the top ranked players. In either case, you will never be in shortage of players to fight in multiplayer. You will never get bored with the different multiplayer maps, and game variants. You may never sleep after you discover halo 3 multiplayer! Interesting array of gamers in this 360 community. From those serving in the armed forces in their down time, to stoners lighting up on 4:20, to kids who curse like sailors, to that white guy that thinks he's eminem.
Story: Couldn't tell you. I completed the story in Halo's 1 & 2 in heroic and they were compelling. Varied difficulty allows for bragging rights on multiplayer. You can play co-op offline or online in the same story line.
Halo 3 vs. Halo, Halo2, Halo ODST, Halo REACH:
This version of Halo has the largest online community out of all of them. If you don't believe me, look it up. Consistent community devoting sleepless nights. Everyone from halo and halo 2 switched to halo 3 because of multiplayer. The REACH beta didn't capture the attention of the Halo 3 community as expected. Should be interesting to see how Bunjie ends up marketing REACH after the flop beta testing.