Although past research has sought to identify the factors of student engagement in traditional online courses, two questions remained largely unanswered with regard to Massive Open Online Courses (MOOCs): do the factors that could influence student engagement in traditional online courses also apply to online courses that are massive and open? What factors do students consider important in terms of their perceived ability to promote a satisfying or engaging online learning experience? This paper reports a case study of three top-rated MOOCs in the disciplines of programming languages, literature, and arts & design in order to address these very questions. Using a mixed methods approach that combines participant observation with analysis of reflection data from 965 course participants, this paper seeks to understand the factors behind the popularity of these MOOCs. Five factors were found. These include the following, ranked in terms of importance: (1) problem-centric learning with clear expositions, (2) instructor accessibility and passion, (3) active learning, (4) peer interaction, and (5) using helpful course resources. The specific design strategies pertaining to each factor are further discussed in this paper. These strategies can provide useful guidance for instructors and are a worthwhile subject for further experimental validation.