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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 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 Standard vanilla Linux desktop
Debian runs standard Gnome, XFCE, KDE - it doesn't use its own special desktop environment or debian specific modifications or customisations, 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 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 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.
Cons
Con Stable release contains extremely outdated packages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.
Con No default support for unfree drivers
Debian does not official support unfree software so if you have a wifi card or anything elese that requires an unfree driver you will need to download the unofficial/non-advertised non-free iso image which contains all unfree driver packages.
Con Systemd is everywhere
Systemd is very hard to debug by itself, and can break in very strange ways (such as not being able to mount a single partition on bootup), but the increasing number of desktops that are relying on specific functionality provided only by systemd (policy kit, dbus, etc) makes the entire system more fragile and harder to debug.
Con Not designed for general desktop usage
By default, Debian Stable is not that great of a distro for general desktop usage, since the packages are very outdated. You'll have to spend a while configuring the system in order to make it work.
Con No choices
Debian basically doesn't give you any choices, even dpkg supports this feature with virtual packages: for example, it is very hard to use a different sound system than ALSA, like OSS4 or to use ALSA with pulse audio emulation, and the same goes for different init systems.
Con Buggy and non-user friendly
You need to know almost as much as Arch and deal with documentation not as good as the Arch wiki in order for Debian to work.
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 Old software is not useful, but is stable
Certainly for the stable and old stable versions. But the testing version has quite up to date software.
Recommendations
Comments
Flagged Pros + Cons
Con Uses SystemD as default init
Debian has support for other init-systems like SysV, runit or OpenRC, however, there is no way to choose a different init-system in the installer, so all Debian Linux installations will default to SystemD.
Con No WiFi for Live System
Pretty much the only live system that doesn't offer ANY of the usual choices for a WiFi connection. Like Network Manager or WICD.
Con Hard to configure
After the initial installation, which is quite simple, it may be hard to configure Debian. Things like printers for example, are notoriously hard to set up on Debian for some computers. Though If you have done this, you will never have to change anything.