Any economic model that does not properly address inequality will eventually face a crisis of legitimacy. Unless the relative economic roles of the market and the state are rebalanced, the protests of 2011 will become more severe, with social and political instability eventually harming long-term economic growth and welfare.
This is the conclusion of Nouriel Roubini's post at Project Syndicate on The Instability of Inequality. It's a good short read. Roubini even quotes Marx, who is appearing more and more in the mainstream these days.