Heres a list of main features I've compiled from the /r/xboxone post

• They decided to give away all future maps for free to not divide the community.
• completely new "epic" multiplayer mode to be presented at E3. They are working on this mode for more than 3 years. Sounds like a huge Big Team Battle mode. It does "incorporate all aspects of the Halo sandbox for the very first time ever".
• Drop-in/drop-out 4 player co-op in campaign.
• in singleplayer, you can give your AI mates strategic commands.
• individual HUD for every team member in co-op mode.
• campaign levels will have many alternative paths.
• Xbox Live Gold for online co-op not needed.
• the Beta was a fundamental base for many positive changes.
• Whether playing as Chief or Locke, you're accompanied by a fireteam of 3 A.I. which you can command/issue orders to in the campaign
• Highlighting an enemy and hitting up on the D-Pad orders your team to assault an enemy
• Other commands include picking up certain weapons, using turrets, driving warthog, etc
• AI teammates can revive you as well
• Confirmed: Linda, Kelly, and Fred join the Chief
• Game starts out before Chief is on the run
• Audio has been heavily reworked from the ground up for Xbox One (bullet sounds, debris, etc)
• 8 months after Halo 4 ends, cataclysmic events are tearing apart star systems, these are tied to the Guardians (Forerunner entities shown in the first teaser trailer)
• Game Informer played the second mission, taking command of Chief and Blue Team to take back an ONI research vessel abandoned for 2 years
• Locke's team is described as having more advanced tech than Blue Team, members use the "ATS" (Artermis Tracking System) on their armor to bring out points of interests in levels
• Team banter replaces interactions with Cortana to flesh out the characters
• There are multiple routes to take in levels/multiple ways to tackle objectives
• Difficulty scales to the number of co-op players
• The zealous religions Covenant are slowly abandoning Jul M'dama's leadership in Halo 4