• Halo 3 keeps track of which levels you’ve beaten in both solo and coop, and on which difficulty. You can skip around and beat different levels on different difficulties if you really must.
• The first player in the lobby (read: the party leader) gets to play Chief; second is The Arbiter; third and fourth are just another couple of Elites named N’tho ‘Sraom and Usze ‘Taham. They’re all identical except for starting weapons (the Elites get a Cov carbine and plasma pistol or rifle). That means that Elites can no longer constantly cloak (but you probably knew that).
• I’ve seen that you can start as an Elite and die and respawn as Chief. I think this happens your friend who’s playing Chief dies at around the same time, so you get to take a turn.
• You can’t respawn until it’s “safe”, meaning someone’s out of the combat zone. With a few players that’s easy, but with one it’s a little more annoying, since if you die your friend has to stop fighting and run away to respawn you.
• Players can only join between levels; once you’ve started a coop level no one can join…
• or leave. So make sure you have the proper amount of time set aside to play, because if one player leaves during coop the entire game is quit and you’re all screwed.
• If one person finds a skull, everyone on your team gets credit — just make sure you finish the level because you can’t save and quit a coop game. (See above.)
• Do your friends a favor: don’t play with a bad disc. As we learned tonight, the (up to) three other players on your team will actually play through the game in extreme slow-mo if one person’s disc is screwed up and caught in a nigh-endless loading cycle. (Obviously this doesn’t happen on multiplayer, they’d just be booted.)
• Update: Oh yeah, metascoring. Just because you can set metascoring to team doesn’t mean that if your team breaks 15k (or higher) you all get the achievements. You still have to each get more than 15k, which is difficult when kills are spread across four people. So if you need those achievements, make sure party leader equips some skulls.

Have fun!