Tone, timbre etc. Don't really care much about range. Some people are almost painful to listen to because you can literally hear the bumps developing on the vocal chords. Lets say, Rod Steward for an extreme example, I can't listen because it hurts to listen. This whole 'soandso many octaves range' is pretty tiring.
If you can be all that you can be in 2 octaves- good on you. You can have 4.5 octaves and might not be able to convey anything through that.
Vocal acrobatics used to be the deal a couple of hundred years ago, luckily we're kinda beyond that.
Tone. Too many vocalists who are considered some of the best of all time tend to over sing and butcher nearly every song. Actually, from a technical vocal training stand point, most teachers consider artists like Christina Aguilera to be bad vocalists. I'm not saying I agree, but I've read it many times. I think Christina has a great voice but simply doesn't have the technique or restraint.
I was reading your post and then I see this on twitter: Celine Dion: "Christina Aguilera is probably THE BEST VOCALIST IN THE WORLD" - On the @tyrabanks show
Tone, I hate when people say someone can't sing because they don't have a huge range, you don't have to have huge range to be able to sing, you may not be a powerhouse, but it's not the same as not being able to sing.
I agree with what Travis said about Madonna, she doesn't have a huge voice and she never will, but she really makes the most of her voice, I could name a few other who are the same too, yet are said to be weak vocalists.
Diana Ross and Janet are good mentions. Neither are powerful but both have done wonderful things with their voices on a multitude of musical styles.