I'm taking powerful as "emotionally powerful". To me, vocals are typically like 80% emotion and 20% technical quality. People like Christina Aguilera may have a technically good voice, but it sounds "cold" to me. I'm sure part of it is the type of music and the fact that many times they don't even write their own music. They might be singing their ass off, but it still sounds insincere to me. The opposite of this, IMO, is someone like Jeff Mangum of Netural Milk Hotel. You'd never say he has a technically good voice, but he's a phenomenal vocalist. An example of someone that has it both, to me, would be someone like Thom Yorke or Bjork. EDIT: And as far as good vocals go, again, IMO, often less is more. Belting something out at the top of your lungs doesn't automatically make it more powerful. Also, voices like Whitney Houston's or Freddy Mercury's bore me... I'll take interesting over "good" any day.