In the last decade, numerous student assignment systems have been redesigned using input from economists in the large American cities and elsewhere. This article reviews some of these case studies and uses practical experiences to take stock on what has really mattered in school choice mechanism design so far. While some algorithm design details are important, many are less practically important than initially thought. What really matters are basic issues that market operators in other contexts would likely be concerned about: straightforward incentives, transparency, avoiding inefficiency through coordination and well-functioning aftermarkets, and influencing inputs to the design, such as applicant decision-making and the quality of schools.