Whenever I have done my ordinary work (economic analysis, writing, research), I have asked for feedback from trusted individuals (and offered it in return for their work). The feedback almost always improves our work.
Now that I have become more involved in theatre, I can see the same thing applies in theatre. Getting feedback is a tremendous help. Directors provide considerable feedback for the actors, but it is especially difficult for them to do so if they are acting in the production and worrying about many other things as well.
The better productions in which I've been involved have brought in "play polishers" to provide feedback and critical reaction (e.g.: the recent production of Cabaret used at least two different play polishers at different times). It has been tremendously helpful, and I can only begin to imagine how it would have helped other productions. Indeed, several times I have had reviewers make comments that would have improved my own work (and in fact did, but alas only after the show had opened). Many of the points made by reviewers would often be made, pre-production, by play polishers.
Even if I never do another show the rest of my life, I hope others in the future will take the step of bringing in "play polishers" to provide pre-production feedback.