You have to give Miller credit for winding up as a more interesting writer than most of the playwrights of his general background and views. All My Sons is basically a Lillian Hellman type of play -- a well-made three act melodrama, like Ibsen rewritten for the Popular Front. With the success of that play, Miller could have coasted on self-righteous melodramatic cliches for years, the way Hellman did; instead he tried to do different things and actually challenge his audience a bit in terms of stagecraft and even theme (Death of a Salesman may be somewhat simplistic, but it doesn't have the black-and-white morality of, say, The Little Foxes). His writing took a downturn in the '60s when Broadway was crumbling and there were fewer directors and producers like Elia Kazan or Harold Clurman, guys who could make a playwright's work better by telling him what to cut and rewrite. Once Miller was an Institution, impervious to such orders, his work declined accordingly. But that's true of most of the playwrights of his generation.
I think his reputation has suffered because one of his silliest plays, The Crucible, is constantly taught in school as if it's some kind of deep political tract. A while back I speculated that being forced to read The Crucible is what turns kids into Republicans; the speculation is at the end of this post, where I wrote:
By the way, speaking of The Crucible: you know how people sometimes wonder why college students are more right-wing than they used to be? I blame the practice of teaching The Crucible in high school. Teenagers suffer through that play, along with a mess of commentary about how in the '50s there was a dark and evil conspiracy to hunt Communists, yada yada. Then they get to college, start reading on their own, and discover that Communism was evil, the Rosenbergs were guilty, etc. Then they're so disillusioned with everything their teachers told them that they go overboard and start campaigning for a flat tax and wearing business suits with little U.S. flags on the lapel. So if there are any teachers reading: if you don't want your students growing up to be Republicans, I beg of you, don't teach The Crucible. You'll thank me later.