bear88 said: "Out-of-town tryouts for original musicals, and I have seen my share at this point, tell me a little but not necessarily a lot. Local reviewers tend to be polite. Same with audiences. Everyone just overlooks serious flaws in the book, score, direction, acting, or all four. It's hard to succeed on Broadway with an original story and no stars, but movie adaptations are no guarantee either. Otherwise, a number of shows in recent seasons would be hits."
I agree with this, but with one minor caveat: it depends a bit on where the out-of-town tryout is given the critics available in different markets. Chris Jones in the Chicago Tribune and Peter Marks in the Washington Post tend to be more critical and address the issues that you note many other reviewers do not. I'm less familiar with the Boston critics for shows like those starting at A.R.T.