The Barnabas character just had so much pathos, emotional resonance, creepiness and evil when he was originally introduced. The fact that he was lonely and vulnerable in spite of his bad deeds makes him so interesting and compulsively watchable. There is just so much going on with him on so many levels in the 1967 storyline. You never know what he's going to do next!
I know that JF has said he was flying by the seat of his pants early on and sometimes it shows. Obviously he was a lot more comfortable with the character by the later storylines. But that nervousness and hesitancy actually worked in his favor. When I go back and watch those earliest episodes, he blows me away! It's also so interesting that he (and the writers) introduced with this character a new horror archetype: the reluctant vampire.
One of the best villains of the small or the big screen - hands down!