I might be the only one thinking this, but on Indestructible and Asylum...his voice seems somewhat stronger...more masculine, to put it this way. I feel like it has a lot more force. And that is one of the reasons why I love those albums that much. Now, that might mean that he's putting even more strain on his voice, which leads to him getting sick faster.
I think somebody said at some point that the Indestructible tour had more concerts in a shorter period of time than Metallica's Death Magnetic tour. Which is ....wow. Metallica had a two year tour! Disturbed sang more in less time which I find amazing and quite shocking, that they were able to do that. Truly amazing.
I had the pleasure of seeing both Metallica and Aerosmith live last year and both Hetfield and Tyler still sound amazing. Of course, their voices have changed, it's inevitable over the years, but they haven't changed in a bad way. They still sound absolutely amazing.
I feel that David can have a career at least as long as those guys. He has the voice and the potential and like I said, to me, his voice is getting more interesting. But he needs to be careful and take care of his voice and take it easy with the singing.
And I agree, I also have unlimited respect for this man as a singer and a person.