There was a poll to determine the most deserving directors, actors and actresses who never won an Academy Award; the polling for the actors was obviously rigged or at least not taken seriously (Demi Moore number 1?), but the list of deserving directors is a pretty good one, except maybe Ridley Scott. In fact, the list barely scratches the surface of great directors who never won an Oscar; there's also Ernst Lubitsch, and Preston Sturges, and Robert Altman, and Sam Peckinpah, and Raoul Walsh and Orson Welles and Rouben Mamoulian and so on. You could make a very good case that the list of directors of American films who didn't win an Oscar is more impressive than the list of those who did.
But here's the thing that always puzzles me around Oscar time: why is there a Best Director award at all? The arrangement by which the Best Picture award goes to the producer only is one that doesn't really make much sense. (In his interview with Hitchcock, Francois Truffaut assumes offhand that Hitchcock got a statuette when Rebecca won Best Picture; Hitchcock corrects him, pointing out that the award went only to the producer, David Selznick.) It was set up that way because the producer was considered the most important figure in the making of a film -- which, in late '20s and early '30s Hollywood, he usually was -- and because they couldn't figure out how to give a "Best Producer" award; after all, you can't separate the contributions of the producer from the finished film, because it's his job to supervise the making of the film.
Well, that's also the director's job. Even under the old studio-system arrangement, where the producer supervised most of the behind-the-scenes stuff (budget, script revisions, staffing the picture, editing the footage), the director was still in a supervising position, coordinating all the stuff that went on on the set, making sure all the elements worked together to translate the script to the screen in the best way possible. Some directors, particularly those who worked as their own producers or writers, did more than that. But whatever a particular director does, the obvious fact is that the director doesn't have a single, easily-definable contribution; his job, rather, is to supervise the contributions of all the other people and make sure they're as good as they can be.
That being the case, giving an award for best "direction," separate from the film itself, strikes me as pointless. Writers, actors, costume designers, composers; all these people can get an award for their work because it's only a part of the film. But the director, like the producer, is supposed to make sure that all these parts coalesce and add up to a good film. If the director didn't make the best movie, then how can he be the best director, when the director's job description is, basically, the job of making the best movie he can? The quality of "direction" is often defined as some visual flourish or cool camera angles, but that's hardly the most important part of what a film director does. If a director creates a lot of great camera angles, but the acting is weak, then I don't think you can say that he did a great directing job in spite of the bad acting; we need to ask, rather, why he didn't get better performances out of the actors (maybe he was too busy with the camera angles). The weak performances in, say, Hitchcock's The Birds are a failure of the director, not just of the actors, and that means that The Birds is not a particularly well-directed film no matter how many great shots Hitchcock pulls off.
I guess there are some cases where you can separate the director's contribution from the film -- like, say, when he was stuck with a bad script which the producer wouldn't let him change; the director might do good work as far as the script allows. But those kinds of movies, B movies mostly, don't get nominated for Oscars anyway, so the point isn't really relevant. When it comes to the kinds of movies that do get nominated, the quality of the script is usually part of the director's responsibility, just as it's the responsibility of a magazine editor to get the best possible articles out of his or her writers. It seems to me that it would make much more sense to just junk the Best Director award entirely and give the Best Picture award jointly to the two or more people who are supervising all the other people on the picture: the producer(s) and the director.