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Debian is composed of free software mostly carrying the GNU General Public License. The operating system is developed by an internet collaboration of volunteers aligned with The Debian Project.
SpecsUpdate
Pros
Pro Comes with over 37,500 packages
Because of its popularity, Debian has a lot of applications available which range from productivity programs to business software, games and development tools. It comes with over 37,500 packages (software that is precompiled and ready to be installed on a local machine) -- all of them for free.
Pro Extremely easy to find help with any problem
Debian is one of the oldest and most popular distros out there. Debian's popularity means that you will always be able to find a solution for your problem just by searching on Google, or if by chance nobody has had the problem you are having it's very easy to ask the community and quickly get a solution.
Pro Standard vanilla Linux desktop
Debian runs standard Gnome, XFCE, KDE - it doesn't use its own special desktop environment, which means that users benefit from the work of the whole Linux community, Debian developers can focus on the distribution itself, and any support for your desktop environment on other distributions should work on Debian as well.
Pro Wide choice in desktop environments
Debian offers stable and testing CD images specifically built for GNOME (the default), KDE Plasma Workspaces, Xfce and LXDE. Less common window managers such as Enlightenment, Openbox, Fluxbox, GNUstep, IceWM, Window Maker and others can also be installed.
Pro Live install images for CDs, DVDs, and USB thumb drives
Debian releases live install images for CDs, DVDs and USB thumb drives, for the i386 and amd64 architectures, and with a choice of desktop environments. These Debian Live images allow the user to boot from a removable media and run Debian without affecting the contents of their computer.
Pro Comes with over 55.500 packages
Because of its popularity, Debian has a lot of applications available which range from productivity programs to business software, games and development tools. It comes with over 55.500 packages (software that is precompiled and ready to be installed on a local machine) -- all of them for free.
Pro Live install images for CDs, DVDs and USB thumb drives
Debian releases live install images for CDs, DVDs and USB thumb drives, for the i386 and amd64 architectures, and with a choice of desktop environments. These Debian Live images allow the user to boot from a removable media and run Debian without affecting the contents of their computer.
Pro Easy to find help with any problem
Debian is one of the oldest and most popular distros out there. Debian's popularity means that you will always be able to find a solution for your problem just by searching on Google, or if by chance nobody has had the problem you are having it's very easy to ask the community and quickly get a solution.
Cons
Con Ridiculous package splitting
Even though it makes sense to split devel and the actual binary of an application, the splitting has become as mess in debian and its derivates: for example the nvidia driver is splitted into over 40 different packages: https://packages.debian.org/en/source/sid/nvidia-graphics-drivers
Con No good choices of fresh desktop environments
GNOME is quite outdated and buggy;
KDE is too old, even in Sid it is at 5.14 branch (at Jun 13 2019 Plasma 5.16 is available in many rolling/fresh distros);
XFCE behaves like crooked, visual artifacts at the taskbar and so on;
Cinnamon as far as I know works better in Linux Mint compared to (almost) any other distro;
For other desktop environments (e.g. MATE) can't say - these are too new (as projects) and possibly incomplete, thus might have many bugs.
Con Stable release contains extremely outdated packages
If you want the newest packages, you'll have to do a minimal installation of Debian stable then upgrade to testing or you need to install testing directly: https://wiki.debian.org/DebianTesting.
Recommendations
Comments
Flagged Pros + Cons
Pro Rolling or Release based
Every Debian Release can either use a Rolling or Release based model.
Out of Date Pros + Cons
Con No default support for non-free hardware
Non-free hardware will not work upon install of Debian and the user will need to search out the correct non-free software to install in order to make them work. This can be difficult if one of the things not working is the WiFi connection.