I never really listened to bands with death growls before getting into Opeth. That's why it took me so long to listen to their other stuff after hearing Damnation (Just my own self professed musical ignorance coming out. I loved Damnation and thought that if their other stuff were as good, I would buy it all sight unseen, since Damnation was such a great album. But I just couldn't understand how a band with "Cookie monster vocals" could possibly be taken seriously).
Now, however, I've come to really appreciate Mike's death growls, especially on the older stuff. It's a part of their overall sound and they use it very effectively, especially when they use the brutality of the growls to contrast the beauty of the softer parts. It's a part of what makes Opeth...Opeth.
Personally, I would hate to see the growls go. Yes, Watershed contains less growls then the older stuff. But I think it's just a case of using them when they best fit the songs. I would also hate to hear an album filled with nothing BUT growls. It's that use of contrasts that gives the band such a unique sound. Like Per once said, the sign of a good musician is that he not only knows what to play, but he knows what NOT to play.
Maybe the songs on the next album will be more conducive to using more growls (Although seeing the overall course of Mike's writing since Blackwater Park makes me doubt that, as he has been leaning towards clean vox more and more). And I would like to see more of that contrast between heavy and light. Too many growls and it becomes overkill...repetitive and boring. But too many clean vocals and it loses that special and unique sound that is all theirs. Yes, I still love Damnation and the songs and arrangements on that album can easily be placed up against the songs of any other band today. But, when compared to the contrasts found on the other albums...I'm kind of glad that Mike said Damnation would be a "one time thing."
It's all about finding balance and contrast. That's what makes Opeth so special. The growls make the soft parts sound that much more beautiful and the clean singing makes the heavy parts that much more brutal and intense.
Just my $0.02.