Debian has a great package management system, called apt this alone makes it a great distribution.
It resolves dependencies while installing software which removes a great headache for starting users. That's probably one of the reasons the immensely popular ubuntu was based on it. (and many other distributions)
Debian goes trough a great effort to create a very stable OS and it has a very thorough testing scheme before releases. This makes it one of the most stable linux distributions.