I think that it can be argued with some success that the central question that Smith sought to answer in The Wealth of Nations was how to envision a capitalist system that would inherit the economic mantle from mercantilism. Smith's desire was to provide an economic study in which the Enlightenment principles of freedom and lack of control could enable an economic system where great potential and possibility for human reason could take flight. The central question for Smith existed in how to conceive of an economic and political system in which liberalism could be enhanced.
For Marx, the central question in Das Kapital centers on displaying the dialectical materialism which will progress past capitalism into something beyond it. The central question for Marx focuses on revealing how capitalism has constructed a setting in which its own nature will reveal its own defeat. Developing this through Hegelian dialectics merged with economic reality reveals an order in which “Capitalist production begets, with the inexorability of a law of Nature, its own negation.” This is the central question that Marx details in his work.
The differences between both central questions reveal points of view in which both thinkers are fixated on different emphases in social and economic theory. For Smith, the focus on examining the virtues of liberalism is where his central thesis lies. In Marx's world, illuminating a dialectical materialism in which capitalism will give way becomes his critical question. Both explore economic and social reality in vastly different manners.