Thanks moutarde, I've read the book some time ago (it is a nice book, though I never thought of linking it with fantasy world-building, that's new). But my hunch is that in most "normal" situations the energy availability supersede the food question at a certain point (the point in which technology explodes), because the most "efficient" ways of producing large quantities of food involve a lot of energy, both in the form of machinery and chemicals.
I don't want to make all the world food-scarce to explain why they are not developing technology. Also, I am not specifically looking into how a non-developing civilization would look like, that is too complex a question, and one into which both casual events and supernatural forces would make a lot of a difference. I would just be content if I can give a semi-credible explanation of why a world cannot reach a technologically advanced state.
If it has just a minimal degree (still fantasy after all) of plausibility, I would like to pursue the fossil fuel unavailability hypothesis.