CafeHayek brings a great response to Paul Krugman's column dismissing fears of debt.
The problem with public debt is that governments that borrow impose on other people - future taxpayers (many of whom don’t vote in today’s elections!) - the obligation to repay. As a result, governments tend to borrow excessively and to spend the proceeds carelessly. The costs to society of the resulting misuse and misdirection of resources are not in the least reduced by the fact that the debt is held internally.
It's not the money, but the poor use of resources, that scares people.
People who know those resources will be needed in the future.