According to Merriam-Webster Online, pluralism is
a : a state of society in which members of diverse ethnic, racial, religious, or social groups maintain an autonomous participation in and development of their traditional culture or special interest within the confines of a common civilization or b : a concept, doctrine, or policy advocating this state.
Basically, pluralism is based on the idea that people within a state have many different values, and we would arrive at some form of homeostasis if we all compete in the marketplace of ideas. Pluralistic governments allow for right and wrong to duke it out in public in order that a stable compromise be reached. The point is not to find the best laws, government, etc. but a state of compromise that everyone can somewhat agree with.
In the United States, pluralism has functioned as a means of applying utilitarianism to government. There have, however, been various critiques. The first problem which arises is the inherent inequality among interest groups. Some interests are more equal than others as a result of American democratic capitalism. Some views get more attention than others. Namely, those with a more monetary drive behind them. As a result, larger attention is given to pro-development messages than conservation messages. Certain groups of people are also more valued than others -- the poor, for example, are grossly underrepresented. The criticism, then, is that pluralism overrepresents some views and underrepresents others, and what results is not a true reflection of society, but a distortion that points out where the money is.
Attempts to correct these problems in the United States have taken the form of sunshine laws and campaign finance laws. Sunshine laws require government to do political dealings out in the open. For example, you cannot have a meeting of the majority of people in city government, or the legislature, etc. that is not open to the public. These laws are designed to open up the political process. Campaign finance laws have been put into place to limit donations to political campaigns, and also limit the political sway of donors.
There is also a microeconomic critique of pluralism, that 1) political players will try to form a minimum winning coalition. The idea of obtaining an concensus is unnecessary, because 51% is all that is required for majority votes. Players will try bargaining and compromise to get a majority, then stop. This does not result in "the greatest good for the greatest number" that utilitarianism strives for. 2) There is an emphasis upon rent-seeking. In rent-seeking, people try to get a benefit for themselves in government, usually at the expense of the greater public. Unfortunately, the cost is almost always greater than the benefit and because the cost is spread out over the population and the benefit is distributed among a smaller number, the public will not be so willing to fight it.