I, personally, hate the idea of having players scale down to the zone lvl (i hate it!), but i would love to have some world quests in old zones, to give those zones more relevant, and, more important for me is, that it can let Blizzard talk about things that are happening right now at old zones without having to modify the entire zone (that will let us know how things changed in a zone after the time passed, will make a zone feel less "stuck in time")