Making all the zones level with you will put a stop to that greyed out quest shit. If you dont pay attention to what you're doing, you'll out level the quests you just picked up. There are long quest lines that are in parts of the game that I have never completed because I would just pick up and move on to a new zone.
I would imagine that its also a good data sample having the zones level with you. So if a player can literally pick from anywhere in that expansion to level, then out of all those zones, the developers know whats most popular. Then they can dive a bit further and find what quests are most popular.
I always enjoyed the look of the Highlands and surrounding area, but I always level over that shit in about 10 minutes. I'll still be disgusted that Im leveling my next character, but at least I'll be passively interested in it now.