It seems like there's always a moment in any Obama speech, especially on race, where you have to just say: "wow - this is new - I've never heard a politician say something this truthful and this eloquent before", and yet, Obama slides these beautiful moments in there so subtly you almost miss them in spite of their sublimity.
In this particular speech it was this moment at 7:50 in this clip:
And because they kept marching, America changed. Because they marched, the civil rights law was passed. Because they marched, the voting rights law was signed. Because they marched, doors of opportunity and education swung open so their daughters and sons could finally imagine a life for themselves beyond washing somebody else's laundry or shining somebody else's shoes. Because they marched, city councils changed and state legislatures changed and Congress changed and, yes, eventually the White House changed.The crowd didn't miss it - they give him a big cheer at that point in the speech.
It really sums up this moment. It's doubtful that anybody in Washington listening to King's speech would have allowed themselves to hope that King's dream would lead to this, in 50 years: the first African American president of the US, commemorating the 1963 March on Washington.