Skyrim - Dawnguard Previews @ Kotaku, Destructoid
Here are a couple of Dawnguard articles, starting with Kotaku who use a bullet-point summary:
The main quest has two different tracks, one for Vampire Lord and one for Dawnguard. Dawnguard will offer 10-20 hours of gameplay, depending on how you play. Even though the two story tracks are separate, they'll come together at various key points in the story. "Whatever the Macguffin is," Howard told me, "both sides come together around that point, and then it diverges afterwards." The demo I played was partway into the Vampire Lord track. The plot involved helping a woman vampire named Serana find her mother, Valerica, who had fled Tamriel to a plane of Oblivion called "Soul Cairn." We performed a ritual to open a portal to Soul Cairn.
...and Destructoid, whose previewer proudly points out he didn't play much Skyrim:
The third-person view of Skyrim was a vast improvement over previous Bethesda games, but you’d still be hard-pressed to find players who prefer it to first-person. It’s baffling to me, then, why the vampire form restricts the player to third-person combat. When I asked the developers about the decision-making process behind this, they sheepishly replied, “Well ... it’s what we did for the werewolf in Morrowind.” Making the same mistake twice is a defining trait of Bethesda, so maybe I shouldn’t be so surprised.
No matter what, third-person will be less immersive than first-person, but that doesn’t mean it has to be bad. Dawnguard is bad because the animation, combat, and movement of the vampire is incredibly awkward -- a few steps back from what Bethesda achieved in Skyrim. The vampire shuffles forward with his hulking body obscuring most of your view; his hand-to-hand combat has a considerable amount of lag on it; and transforming in and out of vampire form is awkward. Then there are other annoyances, like losing the ability to check out your map or manage inventory in the form. Because, you know, vampires are just too evil to stop and read maps.
Platform: PC, Xbox 360, PS3