Awesome character writing.
One thing that really impressed me about this movie was the way the characters are written.
Tiana, for example...she's not a perfect Mary Sue from the get-go; she has her flaws and must learn to overcome them. Namely, the fact that she CAN be a bit of a stick-in-the-mud (no, that little cough of Naveen's wasn't entirely wrong!) and that she is a little bit dense as to what's really important. Her hard work and dedication to her dream is admirable, but from the beginning, she seems to have misunderstood her father's advice to "never lose sight of what's important." (Which even caused a facepalm from Mama Odie at one point!)
Also, I love the way the hero and heroine seem to complement each other in a way that you don't often see in Disney movies. Each has something the other needs...Tiana shows Naveen the importance of being more grounded and responsible, while Naveen shows Tiana the importance of letting go and having fun every once in a while. (A bit of a change from, say, your average romantic comedy movie where the Manic Pixie Dream Girl loosens up the stuffy guy but there's no sense of another side to the relationship.)
Plus, Facilier is a pretty fascinating and three-dimensional baddie once you read between the lines a little bit. Disney acknowledges the racial realities of the time without beating us over the head with them. (For example, the "little woman of your...background" comment.) Once you consider them, Facilier's desire for more power has an extra layer to it. During the scene where Facilier tells Lawrence "Aren't you tired of seeing those fat cats driving by in their fancy cars, never giving you so much as a backward glance?"...look closely at Facilier's face. He's not talking about Lawrence.
(In fact, some fans have theorized that Facilier focuses on Big Daddy, not just because of his wealth and power, but because he's Big Daddy's illegitimate half-brother! An interesting theory...he could be thinking, "I'm as much my daddy's son as he is...why shouldn't I be entitled to the same?")