The Clans were awesome in Halo 2, I honestly feel bad for the people who never experienced the wild and crazy fun from clans.
Clan Battles were awesome. It had its own section in the matchmaking, if your party consisted of like 3 or more people from your clan you could play against another clan in a clan battle. The clans had ranks too.
Peon
Member
Staff
Overlord
Bring the clan ranks too!
Many people I know from Live and other message boards websites want the Halo 2 Clan System.
My group on Bungie.net also is in agreement for a Clan System.
Make it happen! Please!