Cons / Disadvantages of Democracy
Even in theory, democracy is far from the "holy grail" of structures of governing bodies due to a number of serious deficiencies.
Inefficiency and Impracticality: The larger democracies get, the more difficult voting and vote tallying becomes, and the democracies become subject to voter fraud. Also, democracies can suffer from voter intimidation or retaliation, thus taking away from their true democratic nature. The biggest problem, however, of democracy seems less heinous but is actually more problematic: inefficiency. The larger a human system gets, the less realistic it is for each person to vote on a decision. Even in small democracies, there can be a serious problem with keeping people informed of and interested in the issues at hand.
Exploitation of the Minority: In pure democracy, there is nothing to keep the majority from exploiting a small minority. For this reason, the checks and balances of the United States government balance democratic structures (electing officials to congress and the presidency) with non-democratic structures (executive and judicial appointments).
"Least common denominator" rule: Even when everyone is well-intentioned, a poorly informed majority can make bad decisions that hurt everyone. This can be a particular problem whenever there are policies to enact that have subtle and complex implications. Since the majority is by definition not the most highly educated group, education of the masses becomes a limiting factor in the effectiveness of a democracy.
The "Tyrrany of Time" - In organizations with open board meetings run by direct democracy, an interesting problem arises, which is that the people who have the most free time are able to influence the organization the most, for the simple reason that they can show up to more meetings and participate more. People with other responsibilities, on the other hand, cannot. I saw this phenomenon at the Occupy Philly movement, where the meetings tended to be dominated by unemployed people without children, because people working full time jobs, people in school, and people with children were unavailable to attend as many of the meetings.
An incentive for polarization - One problem with democracy that I only became aware of when I started working with consensus-run organizations, was the way majority-rule voting systems create incentives for people to reach out most to undecided, uncertain, or "moderate" voters, and have weak incentives for people to converse with the people whose views differ most strongly from theirs. In consensus-run systems, the incentive is reversed. I think this is a downside to democracy because it leads to increased polarization, whereby groups with opposing viewpoints tend not to talk to each other and tend not to resolve their disagreements or work together.