Rock stable operating system - Linux is very stable, a Linux machine runs several months without having to be restarted with no hangs at all.
Almost no hardware-dependecies - Linux can be used on many hardware-platforms, from an old 386-cpu up to modern supercomputers and 64-bit-cpus.
Professional filesystem - no more windows device-letters, all devices are directly integrated into the filesystem
No forced updates - updates and fixed bugs are available very fast, because new kernel-releases and new program versions become public frequently. However, if you are satisfied with your running system since it functions well, you can kepp it and don`t upgrade at all. Nobody is being forced to upgrade!
Free of viruses, malware - up to today, there are almost no Linux-viruses known, and even if you get one on a system, the danger is quite small, since infected programs are not started by root.
No secret backdoors - there are no "secret" commands that some companies just do not list in their documentation to have market-advantages. All documentation and Linux Sourcecode is freely available for everybode at all times.
Multi User Systems - Linux is a real multi-user-operating system, which means that on one system many users can work at the same time (for example via network) using only the users own rights and privileges that started the program. Using a normal typical fileserver like Novell or Windows NT, this is impossible. Using multi-user-features it is very easy to remotely adminitrate a machine.
Ressource saving - Linux can be started with no graphical user interface, and all of its functions are still working. This makes a lot of sense especially for server-usage since those services run mainly in the background.
Modest hardware-requirements - Linux is not as ressource-hungry as other systems (ram, performance etc), a typical Linux can run on 16 MB ram as fast and performant as other systems with 64 MB ram.
Comfortable terminal-management - Linux graphics system (XWindow) works via network which means that a user can start programs on distant systems and use its graphical interface on his own computer.
Alternative desktops - users can have multiple desktops to work on, spreaded on one or more monitors, and switching the desktop is just one mouseclick away. So if you have too many open windows, it`s still easy to navigate.
System-changes with a running system - Linux doesn`t have to be restarted after system-changes, even if the configuration has changed completely! All you have to do is restart the processes (daemons). Even system-programs can be changed with running systems.
Ressource allocation - Linux not only offers you secure safety guidelines, but also user quotas monitored by the system automatically.
Internet technology - being a full-valued UNIX, it offers you all possible ways to implement Internet and Intranet. Even the most actual add-ons are available like masquerading (NAT), tunneling, IPv6, ATM and so on.
Networkcompatible - Linux can be installed and configured via a network without problems.
Cooperation with other systems - Linux can be used as file-server for Windows computers, so Novell and Windows NT can be completely replaced. In addition to that Linux can cooperate with nearly all other systems via all known network protocols.
Source from: http://www.hardware-chris.de/LinuxSamba_E.html