I would like that every time I encounter a mob it has to a challenge to kill it, and if I pull a second mob it would be interesting that winning against 2 mobs it is almost an impossible task or at least I use all of the class resources or skills. That way it would be interesting trying to avoid pulling to many mobs.
In terms of NPC I would like them to speak every time you walk close. I believe is a nice touch ,for example, to be welcome by the guards when entering to a city.