Greek city-states have their own customs, currencies, laws and militaries. A citizen of an individual city-state is very loyal to his city-state and does not give loyalty to the total nation.
Of all of the city-states, Athens, Corinth, Argos, Sparta and Megara were the most powerful. The concept of democracy resulted because of Athens, but there are some differences when one looks at democracy then and now. During the time city-states existed, citizens had to participate in the democracy or face harsh punishment. Today, citizens have the choice to participate or not, and they do not face consequences for not participating in the democracy.