I was relatively young when I noticed that what I was being taught -- how to pronounce languages, how to articulate notes, always to sing in tune and rhythm, etc. -- wasn't the same as what the most famous singers were doing. I concluded that this meant that they knew something I didn't.
Now in the days of YouTube no one seems able to make this logical leap. The constant carping that is seen in the comments for famous singers seems to come from individuals who studied singing and believed completely in what their teachers were telling them. What follows is my conclusions, not your teacher's.
Maria Callas was a mezzo and was pushing her voice up into a tessitura it wasn't precisely suited for. This and not her weight loss is the cause of her loss of quality in the upper register. If you can't stand to listen to it, find someone you do like. All this carping isn't going to change anything.
Jonas Kaufmann became wildly famous because he found a teacher who could get him to open his throat and produce a heavier, darker sound than the wimpy lyric tenor he was doing before this. He still lapses into a lighter tone on occasion and cannot seem to commit to this dark sound. Sounding like a baritone doesn't mean he is a baritone. Tessitura and not color is the basis for classification. Lesson--please pay attention--which notes you can produce comes from the vocal cords, and what color your voice creates comes from the resonators. Classification comes from the vocal cords, not the resonators. Please don't make me say this again. Sometimes there is a mismatch. Vocal cords always win.
Anna Netrebko sings a lot of coloratura repertoire, but doesn't seem to articulate the notes the way you were taught in school. Lots of singers slur the coloratura: Maria Callas, Joan Sutherland, Edita Gruberova to name a few. I covered this in my I Puritani contest post. It's fine for a student to always articulate cleanly, but no awards are given out for achieving this. When push comes to shove, you had better have more to offer.
Renée Fleming sings virtually everything in a very slurred, Romantic style with very little hard articulation of consonants. Please note that she is very very famous, far more famous than the people who are spitting away on their consonants. She has found her gimmick and does not need your carping.
I allowed myself to get swept up into a YouTube argument by someone who wants to trash Jonas because he used a glottal attack on something in Italian. This subject always reminds me of an occasion when I was studying a piece by Henry Purcell with the words "all, all, all, all, all is love to me." My teacher wanted, insisted on a glottal attack on every "all." I thought this sounded ridiculous and dropped the piece from my repertoire.
Cecilia Bartoli does a lot of glottal attacks. I remember being shocked by this, but there is no evidence that she has suffered any ill effects from doing it, the only thing about it that concerned me. I think the amount of force involved is crucial.
In the German language the word "Einsam" which begins Elsa's Dream would start with a glottal stroke. Because this is normal speech. If I'm wrong, please correct me. Just be sure you actually are German first. Some singers do this, some don't. Does it form a basis for hysteria either way? I think not.
When I am judging the quality of a singer's performance, I do not consult the encyclopedia of principles I learned in school. I consult my heart. If I feel the familiar twinge, I know I have found gold.
I feel much better now.