Fallout: New Vegas, to my knowledge, tried something new in their DLCs, namely having an important part of the narrative get resolved through the DLC instead of the core game. This is unique from other substantial DLCs, like Mass Effect 2's or Fallout 3's, in that the material is directly tied to the main plot, instead of tangentially related side stories that don't need answers (or in the case of Broken Steel, a continuation of the story).
I personally think that, strictly from a storyline/experience perspective, this was a great step forward for DLC.
I personally think that, strictly from a storyline/experience perspective, this was a great step forward for DLC.