Bungie has said co-op difficulty in Halo: Reach will be scaleable.
According to Brian Jarrard, speaking to Kotaku, enemies will get harder when there are two or more players involved, when compared to solo-play.
Previous Halo titles required player groups to manually increase the difficulty level for more challenging gameplay, but Reach enemies automatically get tougher and smarter for each player in the group.
Good to know.
Halo: Reach is out on September 14.