Wagner is serious taboo in Israel. He, though a luminary of the highest magnitude, was known to be a staunch anti-Semite. Though he died before the Nazis came into being, he did provide anti-Semitic inspiration for them, for whom he was a cultural icon. Hitler admired Wagner and his music was played at Nazi rallies.
Barenboim, himself a Jew, asked those in the audience who objected to Wagner's music being played, to leave. His audacious statement was cheered by a majority in the audience, though a few left in silent protest. It was a victory over hatred, it was a victory over racism and, ironically, it was a victory over a great deal of Wagnerism.
I'm sure the great man would be put to shame by the clemency of art-lovers who rose over personal inhibitions and racial sentiments to listen to his music, detached completely from the other things he stood for. And that's the way it should be.
Personally, I've never liked Wagner's music.