The Washington Post has a nice obituary here.
Abbado was one of the truly great conductors of our time. Known mainly for opera - he served as Music Director at La Scala and Vienna (at different times, of course) - he was also a wonderful symphonic conductor, especially of Mahler. His recording of Mahler's Fourth Symphony (from 1978) with the Vienna Philharmonic and Frederica von Stade remains my favorite. The Adagio (Third Movement) is gorgeous and heart-rending.
I think I only saw Abbado conduct in person once or twice; my mind is a bit hazy about the programs (though, given my preferences, it was likely Mahler on both occasions). He was not among the flashiest of conductors, but he evoked from his orchestras consistently great performances; and that, after all, is the mark of the true maestro.