Sonically, the LCD-3 is an upgrade over the LCD-2. It is not a $1,000 upgrade, as most have stated. Diminishing returns and all that jazz. Comfort-wise, the LCD-3 is a significant upgrade over the LCD-2. To me, that's what you are really paying $1k for. The comfort.
You can state diminishing returns all you want, and it's obviously the truth, but at the end of the day you're spending 1k for comfort and small sound quality increase (the word small being relative, when in comparison to the price paid for it.) If these were $500 more expensive than the LCD-2, then it would be more likely to be considered a large increase in sound quality.
All things have to be considered in context...but no, these are not a significant upgrade in sound quality for the price paid. I do enjoy their sound more than the LCD-2.2s I own, but I prefer the extra comfort the most.