I think a good death metal vocalist is capable of scaring the shit out of the listener with the eeireness and the demonic nature of his voice. Power is good, but I prefer intelligible vocals, because sometimes being able to understand what is being growled adds to the frightening aspect.
A good range helps. Rather than a monotone constipated dog growl, a good death metal vocalist should be able to bark, scream, screech, and 'rap' per se, or just rapid-fire grunt.
Any death metal vocalist who can do most of the above and also unleash gorgeous clean vocals as well should be considered one of the all time great vocalists. I'm sick of hearing how awesome Robert Plant, Rob Halford, Geoff Tate, Freddie Mercury, and Bruce Dickinson are. Singing all high pitched does not instantly make you good.
See: Mikael Akerfeldt, Frank Mullen, Mike DiSalvo, Lord Worm, Burton C. Bell, Speed Strid, Tomas Lindberg, Max Cavalera, Shagrath, Chuck Schuldiner, Ihsahn(pre-IX Equilibrium), Rob Flynn, Mikael Stanne, Barney Greenway, Dan Swano.