This is a recent essay by Anthony Tommasini in the New York Times about the difference between opera and musicals.
He proposes that the difference is part of the old words vs music argument. In opera it's the music that is most important, and in musicals it's the words.
I guess. Of course, opera isn't a single monolithic medium. I would like to suggest that like everything else, opera is organization driven. An opera company is a bunch of symphony players, singers and dancers all contracted to present musical theater. They are all going to be classically trained, and therefore are going to be expected to present things that suit their training: operas with a lot of heavy singing and traditional sounding orchestral accompaniment. If you went and there was a pit band with synthesizer and miked mumblers singing, you would not be happy.
I find it interesting--I find a lot of things interesting--that the Zurich Opera has two orchestras: one for standard romantic repertoire and another for baroque and classical opera. They don't assume like everyone else that opera is all one thing.
If you are like me and think opera is about singing and not actually about either words or music, the thing that would tell you you were at a musical would be the presence of microphones. We suspect that microphones are creeping into opera, too, but opera companies are more shy about showing it. The gray hairs that make up the majority of the audience for opera do not want amplified sounds destroying what little is left of their hearing.
What Tommasini is saying is that people trying to cross over from musical to opera do not succeed because the music they are creating is simply not good enough to stand up to comparison with Verdi and Wagner. He would have them give up and stay on Broadway. I think I might suggest they try harder.
The heavy hitters like Adams and Glass aren't necessarily doing that great a job either. My strictures would be different. Number one for me would be, "If you don't love at least one opera singer, don't write an opera." And if you do, write one for him/her. Preferably her, but we'll take what we can get. That's exactly how Benjamin Britten became the most successful opera composer of the modern generation.