I'm mostly curious if it will share the same skill set as bows, if it does then no big deal but if it doesn't then that raises the level cap which is always good for an RPG; as to the actual story line of the DLC my guess is it will revolve around Vampires based on the name which is awesome because they where kind of glanced over in this Elder Scrolls installment, I dont think I've fought many aside from the burned down house quest in whatever city that was in. Other theories that seam plausible are a quest revolving around Dawnstar, a mostly useless city apart from two quests and the hidden chest glitch, and a plot line revolving around Mehrunes Dagon and his Mythic Dawn, although this one i doubt due to the fact that he already has the Dagon's Razor quest which is centered around the extinction of his cult
Evil entity is going to destroy the world with their vast amounts of power which they managed to gather without ever being important enough to mention before. You must go kill them. After you kill them, they will never be mentioned again.
There are exceptions, of course, but an unfortunate number of Elder Scrolls plots fall into that description.
From the rumors that i hear it has to do with the Snow elfs or Falmor and some kind of Snow Prince, so maybe the Falmor get pissed off and bring back to life this Snow Prince to help them take back the surface