If Obama's unwilling to use the 14th amendment or any other tricks (and he's messaging very, very aggressively that he is unwilling to do so), then the end result is economic calamity of an unforeseen but definitely terrible level.
The United States dollar is the currency of last resort because US bonds are considered as good as cash -- the assumption is that they have about a 0% chance of defaulting. If they default, it's actually pretty difficult to say what would happen. People would probably sell all their US bonds and buy some other kind of currency or bonds -- China's a popular suggestion, although their current economic situation might give some pause. This would potentially cause a significant loss in value for the US dollar and everything that pegs to it, is tied to it, or is a derivative of it (or a derivative of any of THOSE, etc.).
In terms of America, we would be permanently impoverished as a nation from the collapse of the dollar, the loss of our status as currency of last resort, and our necessarily increased borrowing costs.
Personally, I think that Obama's messaging so aggressively about not being willing to use the 14th amendment that he might be covering up a willingness to use it. This might be wishful thinking, but frankly, I think you'd have to be an idiot to trust the House to handle this at this point.
The cost of using the 14th amendment is that they try to impeach Obama (which fails), tying up the rest of his term. There would probably be political costs as well. On the other hand, the argument would be that Obama overstepped his authority by SAVING THE WORLD, so I'm not sure how effective it would be.