Humm , I heard the same from another member here

However I have a 20 inch and I don't notice any gradation at all , we also have a 24 inch . I do notice between the 2 iMac's the 20 inch does certainly have a more lighter or faded contrast , the 24 is much much sharper and richer .

Then again on the 20 , I have a sunset desktop photo so maybe that is hiding the gradation ?