Are you a vampire? If so, get Dawnguard. If not, Dragonborn.
I kinda didn't like how they reduced the debuffs for vampires. I never played a vampire in Skyrim until Dawnguard, but being a vampire in Oblivion was really quite hard work, avoiding the sun, sneaking into the night to svck someone's blood. Now in Skyrim, all you have is a small HSM recharge penalty