When I heard about the discussion, my first reaction was "It depends on how you define 'standard repertory,'" which was acknowledged as a fair question. The question has now arisen on Bill Burnett's blog Opera War Horses (which you can't access right now) and on Wellsung, where I'm linking to Jonathan's posting and the ensuing discussion in comments.
So just what is the "standard operatic repertory"? Operas the Met's core audience will sit still for? Anything with a melody? Opera War Horses is defining the standard repertory as "a body of operatic works, all of which were first produced in a 140-year period that begins with Mozart’s Le Nozze di Figaro in 1786 and ends with the posthumous production of Puccini’s Turandot in 1926," according to Jonathan. I haven't seen the specific operas named by Mr. Burnett, so no comments there. Anything put on with some regularity at major U.S. and European houses?
For myself, I think 1786 to 1926 is much too limiting. The people at my friend's party came up with Wozzeck as the most recently-composed opera to enter the repertory. Unfortunately, it was completed by 1922, and so Turandot beats it by a few years. I have to wonder, also, whether the party-goers realized that Berg was still to compose, and not quite complete, Lulu, which I suspect is performed about as often as Wozzeck. (I am happy to be proven wrong on that suspicion, though I like Lulu better.)
Moreover, I came up with the following works off the top of my head that were written after Wozzeck and that I consider to have entered the standard repertory:
- Lady MacBeth of Mtsensk, Shostakovich, 1936
- Arabella, Strauss, 1933
- Capriccio, Strauss, 1942
- The Rake's Progress, Stravinsky, 1950? 52?
- All of the operas of Britten, of which I'd consider Peter Grimes (1945), Billy Budd (1951/60), A Midsummer Night's Dream (1960), and Death in Venice (1973) to be "standard repertory"
In my opinion, then, Death in Venice is very likely the most recently composed opera to become part of the standard repertory. (I would love to hear alternative opinions on this point.)
In addition to those works, though, I came up with a bunch of outliers that may or may not be standard repertory. They are mostly American: The Ballad of Baby Doe, Vanessa, and Susannah. I've seen only Baby Doe of that group, having missed Vanessa at Los Angeles and Susannah at Festival Opera in the recent past. Then there are works like Le Grand Macabre, Nixon in China, and even Doctor Atomic that may be standard repertory in another 20 years. I believe that Saint Francois is too esoteric (and too expensive to stage) to make it into the standard rep. There's no way to know now what the performance history will be of works by composers such as Ades and Sariaaho.
But another significant point in this discussion is that the standard repertory changes over time. Opera didn't start with Mozart, but in 1960 how many performances of Handel operas were given? You can hear Rodelinda at three American houses this year, and a couple more in Europe; his operas are seemingly being revived as fast as good performing editions can be produced.
For that matter, Cosi fan tutte didn't really become a standard until the last 40 years. Idomeneo, a great work, was probably not heard at all between the 1780s and the Glyndebourne productions of the 1950s. The only Mozart opera to be performed much from his death until the 20th century was, I believe, Don Giovanni.
I'd go as far as to push the standard repertory back to the operas of Monteverdi, particularly L'incoronazione di Poppea. Operabase reports that it's getting 62 performances in 16 separate productions in the 19 months from January 1, 2005 to July 31, 2006. That's quite a few more than Lulu is getting in the same period.
Of the 19th century operas now commonly performed, Don Carlo seems to have entered the standard repertory with the famous Convent Garden production of about 1958; La forza del destino was performed more from the 1920s through the 1960s than it is today, perhaps because of Don Carlo's increasing popularity. I think Simon Boccanegra is the last great undiscovered Verdi opera; I hope some day it will be performed as often as Don Carlo. La boheme and Madama Butterfly seem never to have left the standard repertory, but La fanciulla del West has yet to enter it.
I haven't said much about operas that were in the standard repertory in, say, 1900 or 1950 that are hardly performed now, but there are plenty, of course. A look through the Met annals or Rosenthal's Covent Garden book will give you an idea of what was then current in the English-speaking world, for example.
Moving forward - it took until the 1960s for Die Frau ohne Schatten to get much attention in the U.S. And Turandot, surely the 800-pound gorilla of all the operas I've listed, was hardly performed in the U.S. between the premieres in 1926-27 and the advent of Nilsson at the Met in 1961. It lived on through the 30s in Chicago, where creator Rosa Raisa lived, and where Eva Turner gave some of her few U.S. performances. Oh, and of course it got some performances in the 50s by San Francisco Opera, with Carla Martinis, Inge Borkh, and Leonie! Rysanek! in the title role. But I don't think any Americans would have considered it standard repertory in 1955; today you can hardly take a step without tripping over the Chinese Princess.