You really shouldn't buy a house without a substantial down payment.
What is considered substantial? If it is 20%, then a lot of people will not be able to afford houses for many many many many years...depending on the area.
20% is good. People should have at least 10% as a minimum. That's the way homeownership was for many many many years...you had to put some of your own money down to get a loan. You had to save and sacrifice first. It was just as hard to come up with 20% years ago as it is now...but people want to live large now AND own a home. All on someone else's dime. Heaven forbid they go without big screen TV's or the latest iphone to save up a down payment. Saving is for chumps anymore.
My own personal theory is that our real estate troubles can probably be traced back to the time a decade or so ago when suddenly you could buy a half-million-dollar (or more) house with no money down. People are more likely to walk away when they have nothing to lose...and when you put nothing down, you have absolutely nothing to lose.