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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 55,500 maintained and precompiled 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 (please note that debian splits every software into many packages so the real number is probably more ~10-15.000 which is still huge.)
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 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 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, 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 Inconsistent init systems
While Linux uses systemd, non-Linux ports use the traditional sysvinit.
Pro Has small amount of today's bugs (e.g. no duplicate web cameras in Skype - likely not Skype's fault)
... But has not that small amount of bugs from 2016-2017 year for long term.
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Con Stable release contains extremely outdated packages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 unstable by editing the repositories. Save yourself time and install a distro that is rolling release by default.
Con No default support for non-free hardware
Non-free hardware will not work with the default set of CDs so the user must download a seperate image with non-free software: https://wiki.debian.org/Firmware
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 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 Adopted Systemd as init system
It's still possible to run Debian with the Sys-V-Init but you will have to jump through some loopholes and you will run into problems with sysv-init sooner or later. I.e, when trying to install k3b, the well known CD/DVD burning software, apt says that k3b depends on systemd.
Recommendations
Comments
Flagged Pros + Cons
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.
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.