I haven't found greed, intelligence, or hard work to be a trait of any one economic class. I have met many people living on welfare with no interest in getting off, trading food stamps for money to purchase cigarettes. I have met wealthy individuals that, when they go through a down turn, get their hair done on credit. I have also met hard working people who are poor and others who are wealthy and many in between.
What I have found to be more common to the wealthy, is financial education. As this is not taught in grade school, it most often comes from parents. I grew up hearing my parents talk about their mortgage, their rental properties, and investments. Many people do not have that advantage. They can learn, yes, but having the foundation is a huge leap ahead.
Read The Seven Sins of Wall Street for a good look at what the financial system can do to hard working individuals without that understanding.
Side note: someone mentioned Naked Economics. The End of Normal is an absolute must-read.