The American Dream is based primarily on an idea of bettering yourself economically, working to get yourself into a better economic position than you started with.
But when you look at the rate of socioeconomic mobility in the US and compare it to other countries, you find that if you’re born poor in the US you’re more likely to remain poor than you are in some other countries (mainly Canada and countries in Europe). Wikipedia section describing it. Obviously there will be countries where bettering your position is even harder, but it seems to take at least a bit of the wind out of the “American Dream, a land of equal opportunity” sails—especially if we want to claim we’re the pinnacle of that sort of thing.
If I recall, diving into the statistics in a state-by-state or city-by-city way reveals that where you’re born/grow up/live within the US also has a significant effect on your chances for moving upward socioeconomically. Your chances are much better in certain areas than others (which, again if I recall, was largely related to the relative wealth or poverty of that region).
I didn’t take the time to review this to make sure I’m totally accurate, so if someone sees something I’ve misstated or something key I missed, etc., please do point it out! But I think that’s the gist.