The conceptual design phase is a fascinating moment to observe how a design task is interpreted, as the (often implicit) relative importance students accord to the various requirements and constraints offers a window into the thinking underpinning their designs. Our qualitative study used the think-aloud protocol with 11 third year computer science students working on a software design task to investigate the criteria that students used to guide and evaluate their developing conceptual designs. While the trio of feasibility, economic viability, and consumer desirability are often used in design decisions, our analysis also looked for how aspects of ethics (i.e. ethicality) and sustainability informed students’ thinking. We found that considerations of feasibility and consumer desirability dominated students’ thinking, while economic constraints were rarely addressed and even less often the economic impact pertaining directly to the software design. Students’ consideration of ethicality in terms of data privacy and accommodations for disability (an explicit criterion in the design task) indicate that many students did not see ethical aspects as sufficiently important to influence their design choices. Sustainability was introduced tangentially in the design task but was absent from students’ thinking and design decisions. Our findings suggest that ethicality and sustainability should be explicitly included in the design thinking model taught to students for software design to ensure that they bring these considerations to their professional work and therefore to the next generation of software.