[nicktelford]

When you use Through-Silicon Vias (TSVs) to connect the layers together, you would start to end up with scaling limits, similar to the problems of elevators in skyscrapers: the more layers you have, the higher the density of TSVs would (presumably) be required.

This is probably not an issue for thermal TSVs, because of the heat spreader layer between each silicon layer, but it would become an issue for power TSVs, as each layer would (presumably) require an independent supply of power.