Everyone should listen to and read as many of his lectures as possible. This one in particular, and the book that was collected from it, The Roots of Romanticism, is of essential interest to anyone who cares about intellectual history, art, music, literature, or philosophy. For me, Berlin is the great thinker of his time. The only thinker who found a way to negotiate a path that reconciled the great philosophical problem of our time. The 'isms' of European academics -- postmodernism, deconstruction, hermeneutics, structuralism - everything from Wittgenstein to Derrida destroyed the traditional meanings of every possible way we think of reality. The entire philosophical school which runs through them is a nihlist project which destroys our way of thinking about the world and nobody ever put it back together. Nobody except for Isaiah Berlin that is.
Perhaps Berlin's great contribution to philosophy was the idea of value pluralism. To us today, the meaning of value pluralism seems as obvious as the noses on our respective faces. But perhaps it didn't before Berlin came up with the idea. Thanks to Berlin, we can now say that I'm right and you're right, even if we completely disagree with one another. Both of us feel we are right and the other person is wrong, yet truth is something on which none of us has a monopoly, therefore we ought to act as though the other person is as rational as we are. It seems obvious, yet millenia of wars prove that it's not.