This is actually an old practice. Up to the 19th Century, audiences would applaud what they liked, and if it was demanded, music would be repeated. In opera, it would be an aria; in a symphony, it would be a movement; etc. Beethoven, for example, repeated his slow movement from his Symphony 3 not once, but twice, because of the high demand. Repeating music was done to please an audience, but also because it was the only time one could hear most of this music (ie, before records, CDs, and so on) in their life. I think Toscanini when he conducted the Met and La Scala instituted no applause during opera (and also his orchestra concerts). From then on, it has mostly been that way. We are slowly seeing a return to the old practice of applause of what people like during concerts and opera. I think it is a healthy development for the audience and performers to have that immediate connection.
Florez LinkML/NJ
I'm not sure. Audiences here in NYC applaud a lot of mediocre performances. And besides they only applauded a minute and a half for Florez last night. Last year when Netrebko opened Puritani she got two minutes after the mad scene and it never seemed to me that there was a chance she was going to repeat anything. (Netrebko wasn't merely very good. She was great; perhaps the best ever.)
Supposedly the only recent time a soloist repeated anything at the Met was Pavarotti singing in Tosca in 1994. I wasn't there but I can assure you that each one of the handful of times I heard him sing this role, he did it better than he did it in 1994.
ML/NJ