Burke Devlin in "Dark Shadows". Mitch Ryan's drinking addiction got him fired but he was perfect as Burke. The guy they recasted as Burke just didn't have much of a personality.

Not a popular opinion, but I don't like killing/putting a character off screen just because the actor/actress screwed up something off show. Recasting gives another actor/actress a shot at depicting a character their way. Of course my example above didn't pay off. I think it worked on "Bewitched" though.