Debian is one of the oldest and most popular distributions among the Linux users. There are probably hundreds of distributions which are based on Debian, or others which are based on distributions which in turn are based on Debian. Although I'm not a Debian developer, I use it for over two years or so, and slowly got to love this OS.
This is not a 'Debian is better than X' article, it's just about Debian alone. So, let's proceed and see what makes Debian so popular and loved among the Linux users.
Stability. I know as a Debian or Ubuntu (or why not, some other distribution) user you probably heard this a million times. But yes, stability. Debian is renowned for the fact that its 'stable' version is always released only if all known critical bugs have been fixed and it's been stripped from most of the ones with a lower priority. When you install Debian stable you have the certitude that it will not give you a headache, that it is secure and that all the tools and daemons will work the right way. There is a reason for which Debian is used by so many web hosting companies as their preferred Linux distribution for their servers.
Huge community. From a user's point of view, especially a tech-oriented one, having enough persons from which at least one will know the answer is a blessing. There are the Debian mailing lists, the IRC channels (both the official OFTC one and the Freenode one), the forums. And why not say it, UbuntuForums.org does a great job too in helping Debian. How's that?, you may ask. Well, Ubuntu is so popular and used by so many people that on the online forums you can get a straight, good answer to a question in minutes. And since Ubuntu is based on Debian, over 90% of the questions which apply for Ubuntu will apply for Debian too.
Good documentation. I'm talking here books and books of official online documentation. We have the Debian Wiki, we have powerful manuals written by professionals, and to list some, here are the New Maintainers' Guide, the Debian Policy Manual or the Debian Packing Manual.To say nothing about all the third-party help sites, news sites, blogs and articles referring to Debian. This makes Debian a very rich-documented distribution, and knowing help is out there if you do a little reading is a key aspect for any user.
Debian Free Software Guidelines. The social contract is a major aspect which affected how Debian developed and continues to be developed. Debian is one of the distributions which follow a very specific code from which there is no straying. Debian will always be 100% free, it will be open, it will fulfill the community needs and it will respect the free software standards.
Debian fits perfectly both as desktop or as web server. With high security and stability standards, Debian makes the perfect fit for a web server. But Debian comes with over 17,000 from which a huge amount is made by GUI applications, desktop environments and many other user-friendly tools.
And the list goes on. The true power of Debian lies not only in the few points I listed here, but in its users and mentality. Software done this way will always be there, because it relies on an entire community to survive, and as long as the community is there, then so is Debian. A proof to this is that Debian has been around almost since the beginning of Linux, in 1993, when Ian Murdock, founder of Debian, announced the availability of Debian on August 16, and together with Slackware or Red Hat, it is at the top of the pyramid regarding tradition in the Linux world.