For an exercise in frustration, first put in all of the sand. It will be quite aggravating to then try to squeeze in all of the rocks, especially near the end. The best approach puts the big rocks in first, then the small ones, and then the sand. By putting the large rocks in first, you assure yourself that they will fit in the bucket. The small rocks and the sand fill in whatever space is left.
Projects fail because they cram more into the design than they can handle.
If a project can keep its focus on the simple story, it will be far more likely to finish on time and on budget. A technology design focused on the simple story will always allow the user to easily recognize how to handle the common and critical tasks. If additional features are added that undermine the basic purpose, the design or the decision to add the feature should be reconsidered. The important work to be supported must always be simple and clear. If it is not, then the risk of the project stalling out due to over-complication rises, and the likelihood that customers cannot work with the technology jumps dramatically.