I hated quests that required killing an NPC because the game felt empty enough already. I would actually use cheats to resurrect NPCs afterwards just to have more bodies moving around.
I will give them credit for giving the NPCs homes and schedules. I love it when games have the NPCs actually live their lives like work, sleep, go to taverns, etc. You lose immersion in a game when the NPCs are perpetually glued to one spot.
That would be cool too. Random NPCs that you only see occasionally. Skyrim did have traveling merchants which was a nice touch. I'd like to see more of that. You could also have people who lived in the country show up on market days in town.