Matchmaking tags such as Questing, Full Act Clear, and Keywarden are being added to public games. Explicit bonuses will be added into mulitplayer mode, giving players XP, Gold Find, and Magic Find boosts. Additionally, monster health is getting dropped down from an extra 70% per additional player to 50% per additional player.
The UI is going to be working in favor of co-op, too. Whenever a player deals damage to or takes damage from Elite packs or Treasure Goblins, a notification will be sent out and the player's map location will be marked for easy regrouping. Private chat channels with up to 99 other users will be made possible.
You can check out the full list of updates on the official dev blog.