Some quests are better with an empty world. The lone hero who saves everyone quests in particular work best when there isn't someone else also around saving the world. Waiting in line just gets to be ridiculous. "Excuse me, sir, but could you hurry things up a bit? I also need to destroy this threat to the world." Kills and spawns can get silly too when you've destroyed the threat to the world, only to have it pop back up again and fight you while you're riffling through its pockets. This gets doubly bad when the next guy in line to save the world has to wait because you've gone and tagged the threat to the world. It's not as if shared kills solve this, as then we end up with a handful of Lone Saviors of the World teaming up to repeatedly kill the threat.

On the other hand, if things are too empty, then quests can get to be too hard. Many quests with elites are of this sort. Many of those are gone. This can still end up looking silly, because the quests were clearly designed for multiple players involved and went from being a significant accomplishment to being lame. I'm looking at you, Jintha'Alor. On a side note, not only was that place tricky to run at level, but it's also the worst Archaeology spot in the history of ever. Quests for mass killing are simultaneously well- and poorly-suited to a high population. On one hand, if those rats/orcs/rarcs are such a problem, then it makes sense that there would be a ton of people hired to kill them. It makes it feel a bit more like a war/magically oversized pest control when you have a dozen people busily killing them. Yet this depends a great deal on the spawn rate: if they come back quickly, then all is good. But, if the respawn is slow, then you have a bunch of guys standing around drinking coffee and yet, none of them claim to be supervising or on break.

Population variance can take a turn for the comical, utterly ruining the quest, but creating an equally-great situation, if only you can see it from the right perspective. For example, Westfall is meant to begin with an investigation into a murder. This should mean the lone player carefully questioning various vagrants, possibly bribing them, a feature that I believe all quests should have. In practice, it means racing people for the opportunity to punch a homeless person. Perhaps not what the developers intended, but it works just as well. And the ragamuffins have a field day.