JOKESnSMOKES:

 WoW can have side 'missions or quests'. If i was to think of any for a Halo MMO, I couldn't.  It all would have to be story line based imo.

 Side quests meaning go kill 10 boars or something rather.

URF (Rebel) incursions, UNSC personnel hostage incidents, Kig-Yar pirate raids, Colony evacuation procedures, Weapons cache gatherings, indigenous lifeforms invading colony settlements... I could go on an on.