The Turn of Screw at BAM. Written in 1954, it represents both the first 20th Century opera I've seen, and the first sung in English.
I was very aware of how different the music was from other operas I've seen, and I enjoyed it very much musically. The singing was wonderful and I found the score intriguing and engaging. However, I found it odd listening to opera sung in English; I prefer the romantic mystery of it sung in another language, particularly Italian.
Most unenjoyable to me, however, was the story. I had read the Henry James novella in my twenties and was perplexed and nonplussed by it. I just didn't get it. The story has a level of abstraction to it that I might be too dense for. In any case, I thought the plot was a poor choice for an opera, and I felt put off by it. Also, the production staged it as taking place in the 1950s, losing the more romantic and gothic quality of the isolated Victorian setting.
I'm really glad I saw The Turn of the Screw, though. The lighting was beautiful and I'm glad to have been exposed to 20th Century classical music.