Not sure if Henri Lefebvre qualifies as a neo-Marxist, or an ex-Marxist, but his theory as to why capitalism has continued past the industrial age is perhaps the most important, and neglected theory.

His theory is that cities store capital, and it is this 'production of space' that creates surplus-value. Other philosophers have gone on to study 'uneven development' of capitalism. David Harvey's early work was taken directly from Lefebvre.

I think what separates the 'neo-Marxists' from the Marxists is going beyond simply studying economic causes, but other social and structural causes.