Oh wow, there are quite a few reasons to use Debian. I've use several distributions, including SuSE, Caldera, Redhat, Mandrake, TurboLinux, and Slackware. Each one of them could not even come close to Debian in terms of stability. Granted that stability comes with the price of older packages, but the amount of thought that goes into the precise details of each stable package is the primary reason Debian is so rock solid.
Another reason is apt. Nothing even comes close to it. I've spent hours in other distributions trying to find the right package, while with Debian I can type one command, and I've got it--dependancies and all. Also, the Debian mailing lists are top notch. The helpfullness and knowledge base of the user community is IMHO the best there is. -Rob