Specifically, the fact that the government debt must be valid as expressed in the 14th Amendment, the broad nature of the debt ceiling law, which allows it to cause the United States to default under certain circumstances, should make the law unconstitutional.
By having the law overturned by the Supreme Court, we would probably get our country's top credit rating restored by the credit agencies, since there would no longer be a possibility of defaulting on our bond obligations. (New debt ceiling law could be constructed that would only cripple our government functioning, but specifically exclude scaring and/or punishing bond holders.)