With that said, whoever wrote the general storyline of Dawnguard needs to handle the entire storyline for the next game. The Dawnguard/Volkihar story is far and away more interesting than the whole Dragonborn concept as a whole imo. Despite being Dragonborn there's this sense of your being "insignificant" to everything going on... even when you're the catalyst behind it. Somehow your existence seems to have relevance in Dawnguard's plotline, and furthermore, Serana is easily the most memorable character not named Cicero in the game. And she's genuinely memorable where almost every NPC in Skyrim isn't.
That is generally how Bethesda games go. Storywise, I'd ranked the Bethesda expansions/DLC like so: Tribunal = Morrowind, Bloodmoon > Morrowind, Knights of the Nine > Oblivion, Shivering Isles >>> Oblivion, The Pitt > Fallout 3, Point Lookout >>> Fallout 3.
Considering how bad Skyrim's main story was, Dawnguard being superior is no surprise (I haven't played DG myself).