I think that Manifest Destiny might have defined us as a people in the first century or so of our nation's history, but it no longer does so. Now, we are defined more by the idea of the "city on the hill."
Manifest Destiny helped drive our westward expansion. It justified taking land from the Indians and from Mexico. It captured our sense of our superiority to non-white (and non-Anglo Saxon) people. It could be said to have defined us as a people because we were an expansionist country driven by our sense that we were better than those we conquered.
But today, we no longer believe in expansionism. We still believe that we are better than most other countries, but we no longer try to impose our will through conquest (Iraq and Afghanistan do not, to me, constitute conquests). Instead, we feel that we are a "city on a hill," an example for the rest of the world to follow. We try to persuade and cajole countries like those of the "Arab Spring" to be more democratic and we try to get China to be better on human rights. We want them to recognize that our way is better and then to follow it. This attitude defines us to a great degree and does so much more than Manifest Destiny does.