Joel's discussion of project managers (MS calls them program managers) and developers got me to thinking about the differences between project managers and developers.
The difference between project managers and developers is where they deal with complexity and decision-making. PMs deal with complexity and decision-making between people. Developers deal with complexity and decision-making in design. From that, the need to deal with people across, up, and down the organization (PMs) vs. the need to deal with technical issues across, up, and down the product change each person's focus. The more the PM deals with the people issues, the less attention the PM can pay to the technical issues. And vice versa.
Even people who aren't empathetic by nature can become great project managers. (I give myself checklists.) But people who can't grasp the technical complexity can't become great developers (or great testers or great architects or any highly technical role). This doesn't mean that people who can't understand technical complexity can't manage highly technical projects. They can — if they understand where the risks are and how to manage those risks. But the ability to understand the technical issues and translate those issues into the people problems — that's the mark of a great project manager.