I am probably too late to contribute much, but I have a few thoughts, for what they are worth--

singing is the most universal element in music, and singing styles vary more than the performance styles of any other instrument, so it is very hard to get any kind of consensus on what good singing really is--To make matters worse, singers can learn most, or all of their craft by intuition and osmosis(even "classical" singers!!), and this is is often a great drawback for them, since they often lack the technical understanding of what they are doing that most other instrumentalists have--and this limits their ability to grow and develop--

Still, some singers seem to take their achievements, their aspirations, and, perhaps themselves, more seriously than other musicians do--without naming names, I have noted that one or two of the contributors have lacked a bit of the humility that tends to run rampant in other discussions--

Perhaps singers need to be more self absorbed and self posessed than other performers simply because there is no button, key or string to push, punch, or squeeze in order to extract the desired pitch--they have to come up with it totally on their own(there aren't even any markings to let them know where to find the right note--they pretty much have to just hope its going to be where they think it is)

It requires a particularly tenacious confidence to stand up in front of a room of people and to believe that, given the diversity of tastes in the world, they will be pleased by the particular sort of noise that you plan to emit...

Even the Three Tenors, with all their talent and popularity, approach the stage with the knowledge that a large percentage of the audience likes the other two better than they like them, and must pretend that it isn't true when they perform--

Those who have anxiety about singing really have apprehensions about the truths that they may be forced to confront when they stand up in front of a full house(no matter how small the house)--and the ones who can't manage it often have no less talent or technical ability than the ones who can--they just have thicker skin--