Honestly, I'm not becoming some disciple of Žižek, even if I seem to quote him a lot. But this from On Belief is really good:

Gilles Deleutze's and Felix Guattari's Anti-Oedipus was the last great attempt to combine in a subversive synthesis the Marxist and psychoanalytic traditions. They fully recognized the revolutionary, deterritorializing impact of capitalism, which, in its inexorable dynamics, undermines all stable traditional forms of human interaction; they approached capitalism with the view that its deterritorialization is not thorough enough, that it generates now reterritorializations—a verbatim repetition of the Marxist claim that the ultimate obstacle to capitalism is capitalism itself, i.e., that capitalism unleashes a dynamics it will no longer be able to contain; far from being outdated, this claim seems to gain actuality with today's growing deadlocks of globalization in which the inherently antagonistic nature of capitalism belies its worldwide triumph. However, the problem is: is it still possible to imagine Communism (or another form of post-capitalist society) as a formation which sets free the deterritorializing dynamics of capitalism, liberating it of its inherent constraints? Marx's fundamental vision was that a new, higher social order (Communism) is possible, an order that would not only maintain, but even raise to a higher degree and effectively fully release the potential of the self-increasing spiral of productivity which, in capitialism, on account of its inherent obstacle/contradiction, is again and again thwarted by socially destructive economic crises.

What Marx overlooked is that, to put it in the standard Derridean terms, this inherent obstacle/antagonism as the "condition of impossibility" of the full deployment of the productive forces is simultaneously its "condition of possibility": if we abolish the obstacle, the inherent contradiction of capitialism, we do not get the fully unleashed drive to productivity finally delivered of its impediment, but we lose precisely this productivity that seemed to be generated and simultaneously thwarted by capitalism—if we take away the obstacle, the very potential thwarted by the obstacle dissipates... Therein would reside a possible Lacanian critique of Marx, focusing on the ambiguous overlapping between surplus-value and surplus-enjoyment. So the critics of Communism were in a way right when they claimed that Marxian Communism is an impossible fantasy—what they did not perceive is that Marxian Communism, this notion of a society of pure unleashed productivity outside the frame of capital, was a fantasy inherent to capitalism itself, the capitalist inherent transgression at its purest, a strictly ideological fantasy of maintaining the thrust to productivity generated by capitalism, while getting rid of the "obstacles" and antagonisms that were—as the sad experience of the "really existing capitalism" demonstrates—the only possible framework of the effective material existence of a society of permanent self-enhancing productivity.