Putting aside my personal distaste, I still don't quite understand the distinction between the singing that's done in musical theater and opera and the singing in contemporary pop music. I looked around a bit on the web for some opinions.
Dave Malloy, a musical theater composer, wrote a very recent essay about the evolution of musical theater. He says that musical theater hasn't succeeded in reaching main stream audiences because it's either trying to be ironic or because when they do rock or pop music on stage, it's the musical theater version of it and not authentic (as in, the composer is trying to write something he/she isn't familiar with). The people who write musical theater scores come from the Broadway tradition and not from rock bands or jazz clubs.
Malloy makes an interesting point that contemporary style singing is about putting the performer first whereas in musical theater, it's about playing a character. I'm not sure I completely understand it. I guess in pop music, the artist is performing to draw attention to themselves, saying "This is me. Listen to me. This is what I want to tell you." Musical theater songs are a moment in a larger narrative and very specific to that moment. Pop music is frequently generic, so people can sing those songs anywhere without context.
New York Times critic, Ben Brantley, in a review of the rock musical Spring Awakening, writes about the difficulty of using pop and rock music in musical theater.
Problems also arise from the challenge of making pop music function as theater music. Pop and rock are best at evoking moods and emotions or announcing attitudes, not heightening a narrative point or defining a specific character. Mr. Sheik's music, orchestrated for a small rock band supported by cello and bass, is often gorgeous in its soaring melodies and gentle rhythms, but its lushness can overwhelm Mr. Sater's moody lyrics, artful and evocative though they often are. And we often seem to be hearing the same notes — yearning, tempestuousness, anger — repeated in song after song; the show becomes saturated in a general plaintiveness that can be enervating.My interpretation of Brantley's comments is this: to move the plot or define character, you need change, which in music is signified by varying the tempo or dynamics, things that aren't really done in pop/rock music, particularly because a lot of pop music nowadays is also dance music. You can't suddenly change the beat in dance music.
In The Guardian, Orlando Gough wonders if opera singers can perform pop and launches into a discussion of how opera singing is different than other types of singing.
He states that in pop and jazz, singers often "throw notes away" (which I take to mean, they don't try to sing every note perfectly). Also, he makes this interesting remark:
Part of becoming an opera singer is about making one's voice cast iron, invulnerable. Of course one learns to do vulnerability on stage, but it's a guise. Underneath, there must be no chink. Folk or pop or jazz is different – the balance between vulnerability and control is always evident in the voice, and the tension is palpable. It's not to do with lack of technique; it's about allowing the vulnerability to show.I don't really understand this either. There are plenty of great actors who are raw and vulnerable and it seems like this style of acting is universally beloved by critics. But we know they are "acting"; they aren't as neurotic or unstable as their character. They're just really good at acting.
Gough makes a point that I do understand. When you perform "standards" like musical theater pieces, jazz songs, operas that have been done many times, you're doing someone else's work and it's more like acting. Whereas when you sing songs that you've personally written or songs that were written for you or music that comes from your culture, that kind of singing comes from a different place.
He mentions the Malian singer Salif Keita who comes from the Western African tradition of singing praise and storytelling. I can see that this type of folk music would be authentic in a way that professionally composed music (like in musical theater/opera) is not. But I don't see why you can't be both raw and technically trained. Going back to actors, you find that most of the critically acclaimed actors who deliver "authentic" performances also went to drama school, worked in theatre, or had some kind of training.
Finally, I found an article in The Chicago Tribune which summarizes some scientific research on the characteristics of a hit pop song. The software they use to predict hit pop songs is not super accurate but it indicates that a simple rhythm, danceability, and "loudness" are key to making a hit pop song today.