[development] Why ARE modules in core?

[Jeff Eaton]

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