I used to sing solo in auditioning classical choirs at university (Bach, church music), but these days, in non-Covid times, it's mostly Irish trad in a seisiún setting, but another huge love of mine is opera, and they all need different things.
Sean nós singing, for instance, features improvised ornamentation, so you'd not necessarily ever sing the same song the same way twice, it's often unaccompanied, and the rhythms are dictated by speech rhythms, and it's not at all 'sweet' or nice-sounding -- you learn it orally, by listening to other singers, who teach in the spirit of handing on a specific regional tradition.
For opera, obviously, you need to train to have a much bigger voice, as anyone who has ever sung 'Happy Birthday' alongside a bunch of opera singers who are used to singing over a full orchestra will testify. And sometimes extraordinary opera singers sound dreadful singing anything that requires something less ornamented and large-scale opera crossover, or opera singers singing folk or jazz can be an embarrassing mess, though someone like Bryn Terfel just about gets away with it, usually. Someone like Katherine Jenkins sounds perfectly pleasant, but has a very bad vocal technique if you consider her from a pure opera perspective, and almost certainly wouldn't be able to sing an opera without amplification which isn't surprising, as I don't think she completed her training.
I remember a TV ad for an Alexander Armstrong album, and the bits in the ad sounded ridiculous to me because he was singing 'Down by the Salley Gardens' and 'Hushabye Mountain' with vocal technique that was still audibly based on his time as a Cambridge choral scholar -- he has a beautiful voice, and people clearly buy his music, but I couldn't listen to something that mismatched.
But honestly, when it comes to singers who aren't classical singers, or who aren't from a specific folk tradition, for me it comes down to someone who is fully in control of the way their voice and its own unique properties, even if, objectively, that voice isn't particularly 'good'. Some people (say, Sinéad O'Connor) are able to translate that to a recording, others (Camille O'Sullivan) really need to be heard performing life, as the voice isn't great in itself, but functions brilliantly as part of a performance...