I like the "fresh soup" analogy that was posted earlier.
To me, green beer is when you can taste every single ingredient in it's own right--like, being able to pick out the Magnum taste, and then the chocolate malt taste in a stout individually. They're their own separate tastes floating together.
A beer is 'ready' when the flavors blend and--without being muddled--contribute to each other to make a whole new experience. So now you can say, man I can really appreciate how this hop contributes to this malt; one flavor flows into the next.
Like chicken soup: you can eat a piece of chicken, a piece of celery, and a piece of carrot one after another, but that's not really an "experience." When you let them blend, mellow, and work with one another, then you have a great soup.