I'm pretty sure multiplayer will be awesome, but I'm also hoping that single player will be better than it was for Halo 2. Also, I wouldn't mind some sort of bot play, like you could face the computer in multiplayer. The bots would probably not be that smart, but would be good to kill some time or to practice when your a beginner.
Counter Strike for the PC has that feature, and I found it useful. Especially the harder bots, so if Halo 3 could implement this, it would help out those who don't have Live yet.