Here's an article about the evening from Phillip Boroff.
Some quotes:
On South Pacific:
“I don’t believe for two seconds that those are Seabees,” Sondheim said. “It’s the happiest war I’ve ever seen.”
“The whole lyric of ‘There’s Nothing Like a Dame’ drives me crazy,” he said. “It doesn’t sound to me like what the character is trying to convey. Where he (Hammerstein) is at his best is in the romantic stuff.”
On Reviewers:
“Musicals are the only public art form reviewed by ignoramuses,” he said. “There are very few of them, I can guarantee it, who know anything about music at all.”
On Why His Shows Do Better in Revival:
“Tastes get more sophisticated as time goes on,” he said. “Sometimes a show grows into its own clothes.”
Sondheim on Critics and South Pacific