In the game "Loki:Heroes of Mythology" you can choose between which type of game you want to begin OR continue. You can play it where the mobs remain dead as you progress through the game OR you can at from any save game, "re-set" the mobs and play the game either way you choose.
To be honest, with the sheer mob density I'm very glad that I can play through and they stay dead. That said, I think having the choice is a nice feature for an ARPG game to have.
Since I haven't been able to play D2 yet (hopefully soon) I'll have to wait and see...but I'm sure there was a design reason for this.
But the idea of certain locations that re-spawn sounds like an interesting idea
ps - I believe 2 Worlds works the same way, as in the mobs stay dead...well at least after you kill their astral spirits after you kill the the 1st time lol
Seems like ARPGs tend to have re-spawns and more conventional RPG games don't.