Ambient Crowd Noises
When you go to major cities, it doesn't really sound as crowded as it may look. This could be influenced by the number of PCs and/or NPCs, but based on population density in areas, there could be a subtle amount of ambient crowd noise. Just one more step towards full immersion.

Those of us who are or will dole out hella money for a sound system might enjoy it a bit more. Alongside the Active NPCs suggestion, it just takes this game a whole 'nother step

World Raid Challenges
YouTube - Aion - giant flying monster
As you can see from that video, it is a giant flying monster and the central concept behind this suggestion. Given it's massive nature, it seems appropriate to kill it with a massive raid. These would not necessarily have to be some of the more important raids, but could be an easier task with less valuable drops. Everyone in the zone would be aware of the monster because it would be visible from any distance within the area.

Basically, you have an offensive target (i.e. whatever you're attacking) and a defensive target. You designate your defensive target before you actually begin to fight so that all heals and buffs automatically target that person without you having to pick them manually each time. Note that it doesn't actually heal them instantly, just targets them for you, so there's no issues if as a healer you have multiple people you're taking care of. It just defaults to whoever you set as the defensive target,