First, here's a story he was fond of telling:
He was a young man in the pulpit in Chicago - this was back in the late 60s or early 70s - and there was a man in town, a slumlord, who treated his tenants horribly. This was a matter of Torah, and Arnold had to preach about it. The thing was, the man was a congregant. Well, Arnold preached, and did everything but come out and name the man outright. After the service, the congregant came up to him. "Boy, Rabbi. You sure gave it to them!"
He marched with King, they said, though I would not hear him talk about until years later ("We weren't brave," he said of those days, "We were scared people doing brave things."). He had fought against the war in Vietnam, and against poverty in the United States, and he spoke - loudly - of the need for a Palestinian state in the West Bank when that was seen as close to treason by most of the organized Jewish community. He was one of the last of the great prophetic social-justice rabbis, and one of the last of the Hillel directors who made the the college campus the location of the most interesting Jewish teaching of the time.
There's a lot by and about him on the Web; zil g'mur, as they say, go and study. But what might not come across is what he taught by the way he taught. It's become a very groovy thing these days for rabbis to lead "discussions" which usually involve letting a couple of congregants say something, and then going blithely on to the point he or she wanted to make in the first place. But Arnold created a space in which every participant had equal access to the text, everyone listened to and responded to everyone else, and such authority as Arnold had came only from his ability to ask better questions and suggest more compelling answers than the rest of us. And so the overarching lesson was that what he did was (in theory) something we all could do, and the existential meaning of a religious teaching within the grasp of anyone who was prepared to do the work.
This belief in the radical competence of each student - which was merely the extension of an overall ethical stance - was combined with a deeply held faith that Torah was important, desperately. It wasn't a game, it wasn't an entertainment. It was about nothing less than repairing a profoundly broken world. More than anyone I've ever known, Arnold seemed to live out the dictum, "It is not up to you to complete the work, but neither are you free to stop trying" - only in his case the work involved breaking down a wall, and the only tool available was his high, hard forehead. And for some reason, there was something about that that made you want to join him. You got the sense that you, too, could make some justice happen, could make the world a little bit better. And you, too, wanted to be part of a Judaism that mattered.
I'm not even touching on who he was to me personally; but that I am a rabbi, and the kind of rabbi I've tried (usually unsuccessfully) to be, has a lot to do with him. I miss him.