(From the Debian FAQ:)
Three key features distinguish Debian from other Linux distributions:

The Debian package maintenance system: The entire system, or
   any individual component of it, can be upgraded in place without
   reformatting, without losing custom configuration files, and (in
   most cases) without rebooting the system.  Most Linux distributions
   available today have some kind of package maintenance system; the
   Debian package maintenance system (6) is unique and particularly
   robust.

Open development: Whereas other Linux distributions are developed by
   individuals, small, closed groups, or commercial vendors, Debian is
   the only Linux distribution that is being developed cooperatively
   by many individuals through the Internet, in the same spirit as
   Linux and other free software. More than 120 volunteer package
   maintainers are working on over 700 packages and improving Debian
   GNU/Linux.  The Debian developers contribute to the project not by
   writing new applications (in most cases), but by packaging existing
   software according to the standards of the project, by
   communicating bug reports to upstream developers, and by providing
   user support.  See also additional information on how to become a
   contributor (14.1).

The Bug Tracking System: The geographical dispersion of the Debian
   developers required sophisticated tools and quick communication of
   bugs and bug-fixes to accelerate the development of the system.
   Users are encouraged to send bugs in a formal style, which are
   quickly accessible by WWW archives or via e-mail. See additional
   information in this FAQ on the management of the Bug Log (13).