What barriers to economic growth can be explained using the Harrod-Domar model?

Definition and outline of the Harrod-Domar model; growth in national income = savings ratio over the capital output ratio. This is put as g = s / k. Define the implication of the model and then outline various impediments to its execution in real life in developing countries. Note that the question necessarily leads to a degree of spin-off in that the basis of the Harrod-Domar equation gives a starting point for discussion of limits to its viability.