Background: Humanitarian engineering (HE) is rapidly emerging in universities and professional workplaces worldwide. In HE, socio-technical thinking is fundamental as HE projects exist at the intersection of engineering and sustainable community development. However, the literature still lacks an understanding of the key features of socio-technical thinking. Purpose/Hypothesis: The purpose of this article is to investigate the key characteristics that distinguish the socio-technical thinking of an expert from a novice in the context of HE projects. Design/Method: We distributed the Energy Conversion Playground (ECP) design task to students starting their engineering degree (n = 26) and practitioners (n = 16). We iteratively and inductively analyzed the responses to develop a rubric characterizing the key features of expert socio-technical thinking. We then scored participants' responses and compared them to identify differences between students and practitioners. Results: The analysis showed that expert socio-technical thinkers can provide high-quality considerations across three domains: "technology," "people," and "broader context." The comparison of the participants' scores showed that both students and practitioners scored highly in the "technology" domain. In contrast, students scored poorly in the "people" and "broader contexts" domains, identifying only simplistic considerations in these non-technical areas, if at all. Conclusions: This study provides novel insights into the development of socio-technical thinking and further validates the ECP as a trustworthy measure of socio-technical thinking. Implications for engineering educators and multiple lines of future research are also discussed.