Green beer is really any flavor experience that you have that you are not happy with that may be present in the first few weeks of the beer's life that is gone when the beer is fully matured, a few weeks later.
The chili/spaghetti analogy is a good one. You know how leftover chilli or spaghetti always taste better the next day or a few days later? The flavors have all come together. That happens over a few weeks in the bottles as well.
It really can be any number of flavors, many of them are the same ones that are on those "off flavor charts." The only real difference is that they will go away with time.
Often like other's have said it manifests as a green apple taste, which is Acetaldehyde, but generally when we referr to a beer as being green it means it's young and hasn't come into it's full flavor profile
yet.
That's why I say don't sweat any flavor or aroma that you experience from yeast pitch day til its been about 8 weeks in the bottle. Then if the beer still has that, you can look at the causes of it from those charts. But still radically sometimes walking away from a beer for 6 months to a year before deciding to dump it, the beer can end up being OK....
A bunch of people posted their green beer flavor experiences in this thread to give folks an idea.
Describe Green Beer.
Those flavors usually dissapear 3-8 weeks after bottling.
The head not being very creamy could be because the beer isn't fully carbed yet and the head proteins haven't full developed.
Watch poindexter's video from my bottling blog. He shows, and talks a little about the proteins developing through the carb process, and lacing and head forming.
[ame=http://www.youtube.com/watch?v=FlBlnTfZ2iw]time lapse carbonation - YouTube[/ame]
Certain grains like carapils, oats and wheat also contribute to head retention and lacing.
The
3 weeks at 70 degrees, that we recommend is the
minimum time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer.
Stouts and porters have taken me between 6 and 8 weeks to carb up..
I have a 1.090 Belgian strong that took three months to carb up.
Temp and gravity are the two factors that contribute to the time it takes to carb beer. But if a beer's not ready yet, or seems low carbed, and you added the right amount of sugar to it, then it's not stalled,
it's just not time yet.
Everything you need to know about carbing and conditioning, can be found here
Of Patience and Bottle Conditioning. With emphasis on the word,
"patience." 
A week or two from now you beer will probably be just as you expect it to be.
