I think apart from basic technical ability and, as Reynold put it, having a face that bears minimal resemblance to a bag of spanners :D, the most important quality in a singer is an understanding of the band's vibe. If a singer is going to convey the emotions and dynamics of a song effectively, they've got to have a really good feel for the music and the lyrics, IMO. Which is why it's good to have a singer who writes some (or all, if they're really good) of the lyrics themselves.