Sonia,
Your post hits home. Most of the major mistakes I have made in communication stem from what Chip and Dan Heath call "the curse of knowledge"--our tendency to forget that others don't know what we know. There is one note that gave me pause:
"High performing organizations put communications at the top of their agendas."
The agenda isn't where it needs to be. It's far to easy to say, "We care about communication," and then create a process for getting messages to those who need to hear them that is fraught with intimidation and the opportunity for reprisal. (Been there, unfortunately.) Communication needs to be at the top of the hearts and minds of managers and leaders at all times. It must be encouraged and practiced. Silence should never be accepted as a response.

Thanks for your thoughtful post!