It is a whole bunch of things most of which have been covered already. That said I would add light and shade to the mix....a clever singer knows light and shade, when and how to use this volume/emotional control to show off the song.
It is easier to say what makes a bad singer than a good one. I hear nasally pop singers all the time that have poor tone, pitch and control......and lack most of the skills mentioned in the previous posts......I especially hate it when you cannot make out the words they are supposed to be singing. They rave about Adele and Emele Sandi.....I just do not get it...they both have voices that annoy the hell out of me....in every way.