We propose that the salient feature to be explained about the glass transition of supercooled liquids is the temperature-controlled super-Arrhenius activated nature of the viscous slowing down, more strikingly seen in weakly bonded, fragile systems. In the light of this observation, the relevance of simple models of spherically interacting particles and that of models based on free-volume congested dynamics are questioned. Finally, we discuss how the main aspects of the phenomenology of supercooled liquids, including the crossover from Arrhenius to super-Arrhenius activated behaviour and the heterogeneous character of the α-relaxation, can be described by an approach based on frustration-limited domains.