What are the primary characteristics of pluralism (also known as interest group liberalism or polyarchy)? What are the main criticisms of this theory?

Robert Dahl argues that interest group liberalism dominants power in the United States.  This is where non-cumulative inequalities, manifested by multiple groups that each have strengths and weaknesses, fight for political power.  Groups form to represent constituencies, where leaders are elected and engage in compromise decision-making within their group and with leaders of other groups. Groups with opposing interests prevent the accumulation of power with one group.
Critics of this theory argue that Dahl only talked about policy decisions that were openly debated.  There is a private or hidden power struggle where it is decided whether or not to consider policy at all. Second, that there are cumulative inequalities where those with a low level of resources are not able to mobilize and represent their own interests. Lastly, a group's leadership does not always represent the rank and file membership of the group as a whole.