The number one vocal skill is actually a combination of several skills and is collectively known as "musicianship". If you are lucky enough to have gained it from osmosis, then God bless you! Most people have no idea why they cannot sing in tune, do great riffs, understand style, form, structure and hear and know melodic and harmonic intervals and more. The audience hears that something isn't quite right but may not know what to call it or how to analyze it. Musicianship goes far beyond merely singing in tune and merely having a great sense of tonality. There is so much more!
Sometimes vocal technique has not been fully developed and there are problems with register transitions, or breaks in tone production quality. Sometimes vocal technique is inadequate for endurance or range or even to sing a specific style competently.
Some singers have problems "finding their own voice" or developing a unique style.
Many singers are musically illiterate; unable to read or write music or to sight sing. Such things can be taught and may even be nonessential, if the levels of musicianship with hearing and executing are sufficiently developed. Knowing chords, scales, chord progressions and being able to write music (or at least to do sequencing) would place a singer in the position of more easily interfacing with musicians. Professional musicians may are usually so advanced as to know and perform such things as second nature. Some musicians think singers are not musicians at all and have little respect for them, treating them (in person OR behind their backs) as if they are severely deficient in musical knowledge.
Recording experience is invaluable and irreplaceable because without it, a singer never has complete objectivity. What do you sound like to others?
For the vast majority, the multitudinous skills of singing do not come naturally and do not come without some help and/or without countless hours of practice.