[development] Why ARE modules in core?

The whole poll discussion got me thinking. At a philosophical level, what DOES qualify a module for inclusion in core? The fact that many other useful things are built on top of it (APIs, etc)? The fact that it provides specific functionality needed by a critical mass of sites? The mere fact that it meets the coding standards? Clearly not #3. :) But obviously, most of the current core modules evolved as a part of the much older Drupal builds. What criteria make something *good* for core? --Jeff