I'm still in the honeymoon phase with my new iPad and intend to write more about it in the coming week. That said, this excerpt from Matt Alexander's review is what I've been feeling.
The device in my hands -- although deceptive in its appearance -- could not have been further from its predecessor in those moments. Both the original iPad and the iPad 2 were defined by their novelty, but this device is characterized by a distinct sense of maturity.
It also feels like a good answer to the question of why someone would or should upgrade. Go read the rest of the article...his writing is exceptional.