I guess to my ears his voice sounds "amateurish" on his earlier stuff. The current tone of his voice and the vocals he does now are very soothing, subdued, haunting and beautiful. With the tone he uses now, even the slightest, most subtle changes in his voice are very effective and send chills down my spine. On the older stuff, he sounds more like any average "Joe Schmoe" who is singing in his room with nothing special or unique in his voice (of course there are some exceptions...like for example "Omerta" and "A Premonition" are very good vocally). I don't know how else to explain it though. I love the music from all of their songs on those albums (except maybe "One Year From Now"...I can't stand that whole "waltz" sound). So by all accounts, if his vocals were as good as they are on NITND and TGCD, I guarantee that one of those two albums would be ahead of NITND or TGCD. But the vocals are the only thing I can't seem to get past on their older stuff.
Jack