Background: Although much is known from educational research about factors that support K-12 teacher professional learning, it has been an ongoing challenge to incorporate these factors into practice in new contexts and environments. We argue that these factors are too often treated like a checklist of discrete elements, either present or not, insufficiently attending to the complexities of design and experience. To understand how Massive Open Online Courses (MOOCs) might support K-12 teacher learning, it is critical to move beyond application of discrete factors to nuanced navigation of the interplay among researcher examination and theorization, designer intention and implementation, and learner use and experience--balancing considerations of learning theory, instructional objectives and specific learning context, and the desires, needs, and experiences of participants. Focus of Study: This study examines MOOCs as a medium for supporting teacher professional learning. What did K-12 teachers identify as meaningful about their participation in the Creative Computing Online Workshop (CCOW), a large-scale, constructionist, online learning experience for teachers? How do the teachers' experiences relate to each other, to learning research, and to the affordances of MOOCs? Research Design: This qualitative, interview-based study draws on 15 semistructured interviews with participants 1 year after they completed CCOW, as well as course artifacts. We used an iterative approach to develop common themes reflecting what teachers found meaningful and key tensions present in these themes. Findings: Teachers described four qualities as most meaningful to their learning: activity, peers, culture, and relevance. Although these qualities were often mutually supporting, three key tensions among the qualities and the implications for the design of online teacher learning experiences are discussed: autonomy, with structure; diversity, with commonality; and experimentation, with validation. Conclusions: This paper challenges the notion that implementing successful professional development for K-12 teachers is simply a matter of following a checklist of design elements. This study presents qualities that teachers found meaningful in an online learning experience, offering heuristics that designers might consider when designing for their specific contexts. Future research might assess to what extent the qualities and tensions identified in this study apply to other contexts, and explore the reasons why contextual changes may or may not influence results.