I think a lot of songs would sound more like Harlequin Forest. Which wouldn't nessicary be a bad thing, as either way, Opeth isnt just about death. They are all around great musicianships. Hell, I bet Opeth could even make a decent techno song if they put their minds to it.
On the other hand, the death vocals as Mikael has mentioned, are the most aggressive vocal style, and thats why they use them. It would be a shame to lose the death vocals on the really intense songs. For example remove the growls from Masters Apprentices and replace it with clean. It's just not the same effect.