Those of you who are familiar with classical singing may remember that when the baritone Dietrich Fischer-Dieskau was new to the scene, he was widely criticized as being boring, flat and uninteresting, and that his voice was “too small” to be operatically viable. Of course, he went on to be one of the most respected singers of the 20th century, in recital and also in opera. He had a highly respected career and was  considered an expert on repertoire and performance, particularly of German lieder (art songs).
If you listen to his early recordings with an educated ear you will hear a great deal of subtlety. He was hardly boring, flat or uninteresting but it is certainly possible to MISS what he was doing if all you were used to hearing was big, broad strokes in loud, powerful singing.
So it is with many things. The small details are surely lost on those who know little or do not have the experience to perceive things at a refined level. If you don’t know much about wine, you either “like it” or “don’t like it”. If you don’t know much about baseball, you wouldn’t know a great hitter from a fabulous closer, or even that there were hitters and closers. If you don’t know art, you might not appreciate someone like Mark Rothko, whose work could appear monotone and, indeed, flat although it is hardly either.
All classical music sounds the same to someone who has never listened to it, and classical singers as well. It takes time to hear a lyric voice from a dramatic one and the style of Handel as being different from that of Puccini. It takes time to hear the difference between two voices of the same weight and color, such that each vocalist’s uniqueness immediately shines out in their sound.
People tend to want to be validated. They want to know that what they do is “right” and “good” and will go out of their way to find agreement with an expert if they like that expert’s presentation or information. I have been approached after teaching or lecturing by other teachers whose work I have seen demonstrated who teach very differently and have different ideas about singing than I do, only to be told by them that they do “exactly the same thing” as me. This is always stunning to hear.
I once watched a colleague who is well-known and very respected present a highly effective master class on music theater which included specific technical advice for many of the singers. After it was over, another colleague (one with only operatic experience), approached me to say it had been “interesting” but that she was disappointed that this master teacher had not done anything “technical”. I wondered if she had been in the same room with me! The master teacher, however, did not discuss “breath support” and “resonance” as much as functional adjustments. If you are not familiar with function, you can miss it entirely, thinking it an unnecessary diversion from the music and interpretation. I was astounded by this, but it was revelatory, in that it was a representation of what’s out there in voice teacher land.
If you are someone who thinks “all singing is the same” and there is only one way to produce sound, you are behind the times. You have to be nearly deaf to think that opera singers sound like rock singers and that the vocal production of the two can be even remotely the same. If you believe that all music should be sung the same way, you are “ear-blind” to stylistic validity and personal expression within a style, and to the variance not only from one singer to the next, but one style to another. If you can’t hear the big differences, you will surely miss the small ones, and that will leave you lost and looking rather foolish if you present yourself as someone who is qualified to teach singing in any style.