My favorite is feedback from the actors who read it. There's something wonderful about the perspective from inside the piece, sometimes you can get an entirely different take on things from the characters' points of view, which is especially valuable.
"Good actors force me to write as honestly as I can. I say 'force' because if you have fudged the truth in a script (and sometimes I want to take a short-cut to the end of a scene or act, a short-cut that the action of the play doesn't justify) a good actor's honesty will shine a very bright light on that moment of chicanery and you will have to fix it or go down in flames. Good actors are merciless that way. They can't help it. It's one of the reasons they're good actors. Mediocre actors let playwrights get away with murder."
- Terrence McNally, preface, 15 Short Plays