One undocumented nicety that I found while questing with my brother last night is that the UI now displays each player's progress on a particular quest. For instance, if you need to collect 99 gnoll paws (feels like it sometimes), when you mouse over the mob it will say something like:
Player A - 46/99 Gnoll Paws
Player B - 38/99 Gnoll Paws
It was definitely a welcome change (assuming it wasn't in the game before in some option that I never looked at).
Are there any other undocumented changes that you guys have caught?