This study presents the results of a meta-analytic study about the effects of digital game-based learning (DGBL) on vocabulary. The results of the study showed that the effects of DGBL on vocabulary learning may vary with game design features (Q = 5.857, df = 1, p = 0.016), but not with learners' age (Q = 0.906, df = 1, p = 0.341) or linguistic background (Q = 0.0001, df = 1, p = 0.994). In light of the research findings, Csikszentmihalyi's (1990) Flow Theory was adopted to theorize the role of game design in DGBL. It is proposed that a hierarchy should exist on the "challenge" axis in the Flow Theory, with adventure-oriented games above non-adventure-based games along the "challenge" axis. The theoretical underpinning is that the dynamic equilibrium between challenge and abilities can function independently of the effects of learners' age and linguistic background.