Disclaimer: all this is from an opera point of view.
IMO singing can be divided into three main inter-related facets: voice, technique and musicianship.
Voice refers to the 'god-given' ability. Size and timbre flow directly from this heading. Range, dynamics and flexibility rely somewhat on this, but (from my limited understanding) mostly technique. The elements of the second section aren't merely absolute, but vary depending on the first (e.g. a large voice is generally less flexible and lower range; a voice with a 'traditionally' beautiful timbre can't be as flexible if it wants to preserve the timbre). This is mostly talent, though bad technique will ruin a voice no matter how beautiful (e.g. Carreras, di Stefano).
Technique refers to things like the quality of the legato, coloratura ability, tone production, vocal evenness, dynamic ability etc. Again, it's interrelated with the other two sections. E.g. large voices = less flexibility = harder coloratura proficiency; poor musicianship = crap phrasing = legato is useless. Technique comes from hard work and having a good teacher/doing research.
The last section, musicianship, can include phrasing, repetory, dramatic ability, tastefulness, and so forth. This is talent (intelligence) as well as researching etc.
While a very small number of the greatest singers ever are near the top in all three categories (Caruso and Bjorling come to mind), plenty of singers fall well short.
Pavarotti I would consider great in terms of natural voice and technique, but third-rate musicianship. Domingo has a less than great voice (compared to the world-class), technique that seems to have lasted for him if nothing else, but a much much more interesting interpreter. You can see in the following clip how incredibly boring Pavarotti is, in spite of his voice, compared to Domingo (this is a very simplistic and biased comparison).
YouTube - Luciano Pavarotti "No, pagliaccio non son" Pagliacci (start about 2 minutes in)
YouTube - Domingo, Cotrubas - I Pagliacci - "No, pagliaccio non son"