Willow Rosenberg did become a very powerful witch by the end of Buffy The Vampire Slayer - in fact she almost destroyed the Earth when she went dark in season six - but there was always something inherently innocent about her regardless of that. With that in mind, it was very weird seeing her as a vampire in the the third season episode "Doppelgangland", as she was a complete deviant and far too sexualised for little old Willow - especially at that point in the timeline!
