Here is a small exerpt for your enjoyment:
Diablo II never had an official, complete design document. Of course, we had a rough plan, but for the most part we just started off making up new stuff: four towns instead of the original gameis one; five character classes, all different from the previous three; and many new dungeons, vast wilderness tile-sets, and greatly expanded lists of items, magic, and skills. We wanted to improve upon every aspect of the original. Where Diablo had three different armor "looks" for each character, Diablo II would use a component system to generate hundreds of variations. Where Diablo had "unique" boss monsters with special abilities, Diablo II would have a system for randomly generating thousands of them. We would improve the graphics with true transparency, colored light sources, and a quasi-3D perspective mode. Level loads would be a thing of the past. The story would be factored in from the beginning and actually have some bearing on the quests. We knew creating this opus would be a big job. Because we had the gameplay basics already polished, we figured we would hire some new employees, create some good tools, and essentially make four times the original game doing only two times the work. We estimated a two-year development schedule.
While not Mac specific in orientation, this piece will be of major interest to Mac gamers, and we recommend that you head over to Gamasutra.com to check it out.