One thing I find interesting, I think it was from the G4TV interview, that mentioned when playing four player co-op in the campaign each player controls their own Spartan (along with their own customisations).
This leads to some interesting questions regarding how the game will allow for this to happen. In Halo 3 Bungie used the Arbitar and his sidekicks in order to give an in-game explanation of why Master Chief wasn't by himself. This was a step up from the Master Clones used in Halo CE and 2. It made sense too since with Halo 3 the co-op number was bumped.
I wasn't particularly impressed by how ODST handled it. The other players were essentially ignored by the narrative. Then again the story in ODST was meant specially for a single player and the character Lone Wolf being alone in New Mombasa.
So these comments about each player in four player co-op in Reach has me intrigued. The comments were pretty clear, in co-op other players will not assume the identity of the other members of Noble Team. They will be their own unique characters.
So will Noble Team be replaced in the Campaign by other player's Spartans? Very unlikely. The game is very strongly character driven from what we've been told. It wouldn't make sense to have the ability to replace the cast. I think this means that the end result will be similar to ODST, there will just be three extra Spartans running around, ignored by the main NPC's and forgotten in cut-scenes.
I hope I'm wrong, I would love to see greater integration of multiple players in the Campaign. Just doesn't seem likely any more.
*Totally ignoring character roles during Firefight in ODST, not Campaign so not relevant.