Design cards are a popular way for designers to encode and communicate design knowledge. Aiming to inform the designers of such design tools we set out to characterize the design space of design cards in a two-pronged approach involving a) a systematic literature survey on the use of design cards and b) card sorting interviews, which were carried out in order to characterize the first impressions of design cards from design students, for different formal qualities and content of design cards. Our results point towards a need to develop more abstract and evocative presentations, that are visually attractive while supporting a flexible application of cards. Future research could explore whether such preferences are consistent with how card sets are used during design processes in practice.