From the conclusion to Arnold Kling's 14th macro-economics lecture:
It is true that markets have been overwhelmed by today's information problems, and consequently resources are unemployed. However, the ultimate solution of where resources belong is not known by government officials any more than it is known by private investors. A large "fiscal stimulus" is, ultimately, a major transfer of power away from the trial-and-error process of entrepreneurial markets and instead toward the bureaucratic planning process. It is not clear to me that technocrats have suddenly acquired the wisdom that would justify such a transfer. In fact, while they may have an air of certitude, they may know even less than usual about the best future direction for economic resources.
A succinct and pithy summary and explanation for why so many economists are skeptical about the benefits (especially in the long-run) of various proposed stimulus packages.