Robert and Elizabeth made for pretty strong stuff for an operetta - a father's mental abuse of his daughter and oppressive dominance of his family. Also, there was a modern, early 60s (therefore, at the time contemporary) sensibility of rebellion through it. Also, musical montage sequences, one in Act One which I would say had a filmic quality (according to what is described in the script). The Soliloquy by Elizabeth which opens Act Two is the equal of any of the greatest of musical theater songs. And in the original production, the finale which had the train arriving, coming at the audience, is said by those who saw it to have been wonderful.
Eminently revivable, but would need money and canny casting, I suspect.