Final answer:
The Athenian democracy was a direct system limited to landowning adult males, while the American system is a representative democracy with universal suffrage.
Step-by-step explanation:
Two key differences between the Athenian democracy and the American democratic system are the type of democracy and who was eligible to participate. Athenian democracy was a direct democracy where adult male citizens could vote directly on legislation and executive issues. In contrast, the American democratic system is a representative democracy, also known as a republic, where citizens elect representatives to make decisions on their behalf.
Furthermore, Athenian democracy was exclusive to adult male citizens who were landowners, which was a significantly small portion of the population. The American democratic system extends the right to vote to all citizens regardless of land ownership, including women and non-land-owning men, following the principle of universal suffrage.