The MAIN difference between DDO and Horizons is in DDO objects in the world can be manipulated. Whether it is shatterring boxes to find a key or pulling levers to open a door or gate or setting a floor puzzle to shut down a force field.

In Horizons you CANNOT manipulate ANYTHING in the world that would allow such things to occur. Thus the Quests in DDO are far more immersive then the ones offered by Horizons. Even with the Quest Editor, the game engine itself would not allow the manipulation of objects. That is why Horizons has no doors on any building that one can open and close.

Thus a Quest in DDO can be more fun and more challenging then any Quest in Horizons as all one can really do in Horizons is "get a Trigger Point" be it an NPC or xyz locations and then kill a Mob or Mobs.

That is why I say Horizons is far more Fed-Ex in its' Quest delivery then DDO.

As far as grinding goes. I only repeated a couple of Quests and still managed to advance my level according to the Quests offered. HZ is far more of a grindfest where you head out to your favorite MoB, pull...kill...rinse...repeat.