Ancient critics of Athenian democracy, such as Plato and Thucydides, argued that the state was dysfunctional because the citizens who ruled it through direct democracy were often too ignorant and irrational to make good decisions. For example, Thucydides claimed that Athens launched the disastrous Sicilian expedition, which led to the fall of the Athenian Empire, because the ignorant citizens had no idea how large and populous the island of Sicily was, and thus were easily snookered by demagoguery in favor of the ill-advised high-risk venture.
It’s also worth remembering that, by modern standards, Athens was closer to being a narrow oligarchy than a democracy. Because women, slaves, and the city’s large population of resident noncitizens were excluded from the franchise, only a small fraction of the adult population actually got to participate in politics (though still a much larger one than in most other ancient states). Athens’ enemies often saw it as a nightmare of democratic egalitarianism run amok. But that was because their own oligarchies were far narrower still.
Virtually no one today would propose reintroducing ancient Athenian sexism and slavery; for good reason. Nor should we try to drastically cut back on the franchise in other ways. But the Athenian system did have the effect of limiting the franchise to people likely to have relatively high political knowledge. Among other things, the main function of the Athenian state was warfare. Most of the male citizens probably had direct personal experience of war, at a time when military strategy was much simpler than today and a low-level soldier or sailor could more easily see the big picture of what was going on. For this reason, among others, Yale historian Donald Kagan argues that Thucydides was probably wrong about the causes of the Sicilian expedition, noting that many of the voters who backed it had probably served on the previous expedition to Sicily a decade earlier, or were personally acquainted with veterans who had.
Though they made their share of ignorant and foolish mistakes, ancient Athenian voters probably knew a lot more about the Sicily and the Peloponnesian War than our voters today know about Iraq, Afghanistan and the War on Terror; or for that matter Obamacare, federal spending, and who controls which house of Congress. In that respect, the dangers of ignorance that concerned critics of ancient democracy are very much still with us today, and possibly much worse than they were 2500 years ago.