According to Aristophanes, what are the problems with democracy as a form of government?

Aristophanes' Acharnians was produced in 425 BC during the Peloponnesian War. The Persian War, in which different Greek city states had united to fight off a foreign invader, Aristophanes viewed positively, as an example of great bravery and a high point in the character of Athens. Unlike the defensive war against the Persians, the war against Sparta seemed to Aristophanes to express the way in which Athenian democracy had moved from a reasonable limited franchise to control of the state by an undisciplined and self-serving rabble and the demagogues that manipulated the greed and ignorance of the rabble for their own ends.

Athens consisted of an urban center surrounded by farms. Aristophanes in the Acharnians represents the viewpoint of the country gentry, who owned and gained wealth through farming, and tended to be conservative and anti-war. Spartan tactics during this period of the war involved attacking the farms and the countryside of Athens every year, but lacking naval power or a decisive advantage in ground troops, the Spartans did not cause extensive problems for city-dwellers or wealthy urban merchants and craftspeople who carried on their business within the city or by sea. 

The policies of the democratic faction included lavish spending on beautifying the city of Athens and daily stipends for citizens who served on jury duty or attended legislative assemblies. Crews of Athenian naval vessels were also paid. In the Acharnians, thus, we see the war as enriching the urban citizens and wealthy merchants at the expense of the country gentry whose farms were being destroyed and who payed certain forms of taxes to support the war effort. This was, for Aristophanes, an example of how democracy can become a "tyranny of the mob". 

A particular target of the play's satire was Cleon, a populist politician with whom Aristophanes had a long running feud. Cleon, for Aristophanes, exemplified the worst of Athenian democracy, a populist demagogue who was corrupt, vindictive, and inflamed the anger of the populace to prolong the war and keep himself in power. The key problems of democracy that he exemplified were its susceptibility to corruption, mob rule, and populism.