Youngest Daughter and I have a running (lighthearted) argument about whether Rogers and Hammerstein wrote operas. She claims they are not, and now she has the weight of a New York Times writer behind her, claiming that musicals are focused on the lyric and operas on the music. You would never bother to listen to "Anything Goes" if you didn't speak the language, but would happily listen to opera in Italian even if you only knew the word "ravioli."
Anthony Thomson is partly correct, but I wonder how he would classify Gilbert and Sullivan? Wild guess: "Despite tradition, it isn't real opera ™".