see, I find it easy to believe that the Coen's might not make a great commentary. Tarantino I have evidence that he can do good ones either with someone (dusk til dawn, hot fuzz) or alone (true romance). but not everyone can do that and knowing what to talk about is difficult.
now that I say that, though, I don't know how great tarantino would be alone on one of his more recent movies. so much of the true romance commentary is him talking about writing it and getting into the business which is fascinating but doesn't relate directly to the film. when you have a great director, alone, talking about their own film... they can talk about stuff on set but that wouldn't quite be satisfying... but then what would be? to have them explain the purpose to each line/shot would get old fast and ruin the relationship you as a viewer develop towards the film. I'd definitely specifically want to have tarantino +... either someone he worked with or a friend of his (like hot fuzz and dusk til dawn are). then again Fincher on Zodiac was good...
ahh. rather than ramble further I'll say: sometimes it's nice to not be able to get exactly what you think you want. and yes I'm saying this to myself as much as anyone else.
but I have to comment on Shandley... listening to him on the screenwriter interview podcast (name escapes me but they mention it on filmspotting plenty) I knew he could carry a commentary because he speaks so personally about his films. It was really refreshing to hear in the interviews and I still need to pick up a DVD of his to hear em. doubt is as good a candidate as any, I enjoyed that a fair bit. not sure what other movies of his have one.
... whew.