Final answer:
Social equality is not a feature of state systems; it is an ideal that states may endeavor to achieve. Other options such as judiciary, taxation, and enforcement are typical features of state systems.
Step-by-step explanation:
The option that is NOT a feature of state systems in the list provided is e) social equality. State systems typically involve population control, a judiciary, taxation, and enforcement mechanisms. However, social equality is an ideal or goal that states may strive for but it is not an inherent feature of a state system. In fact, the level of social equality varies greatly among different states and is influenced by a variety of historical, cultural, economic, and political factors.