Stability

Because software can contain multiple bugs (or features!), a sought after characteristic is 'stability'. An operating system (e.g. Windows® NT or Sun Solaris) being described as stable, signifies that it may be used, as intended, without crashing, freezing or displaying other adverse characteristics.

Selecting an operating system for your primary systems, where reliability is essential, will require a stable environment. Hence the reason why most corporates will retain older versions of systems software to 'allow the bugs to be ironed out' before they migrate to the newer version. Even then, they will often consciously remain at least a 'point release' behind; valuing stability and reliability above all else.