3D promises a new dimension in composing systems by aggregating chips. Literally. While the most common uses are still tightly connected with its early forms as a packaging technology, new application domains have been emerging. As the underlying technology continues to evolve, the unique leverages of 3D have become increasingly appealing to a larger range of applications: from embedded mobile applications to servers and memory systems. In this paper we focus on the system-level implications of 3D technology, trying to differentiate the unique advantages that it provides to different market segments and applications.