With the removal of stats like strength and speed from Skyrim, a great deal of what made vamps particularly powerful in Oblivion is gone. I like to think that their statistical weakness relative to Oblivion has been made up for with their ability to travel around in the daylight unmolested (until their final form). This is a fair trade in my eyes. Now vamps suffer reduced health, magicka, and stamina in the daylight and are unable to regenerate. This, to me, is also fair. That said, I believe there should be other passive abilities associated with playing at night. It should flip between night and day - to make up for the loss of vamp's weapon resistance from Oblivion, Skyrim should have given vamps added health and stamina, as well as added regeneration rates when night falls.
An "enslave thrall" ability would have been nice, too. NPC vampires in Skyrim have human thralls they enslaved using their mental powers. Instead of the weak and underwhelming zombie spell vampires get (that can be bought at any store), you should be able to enslave mortal allies indefinitely. The power of the spell to enslave mopre powerful thralls should increase with each level of vampirism.
Just some thoughts.