Thinking about it more, I realize that the problem with Inception is just that it wasn't imaginative enough. More precisely, the nature of the plot prevented the director from fully realizing the unique logic of the dreamscape. We experience dreams as variations on reality, in which the laws of physics and of narrative can be suspended, sometimes at will and sometimes because of exogenous shocks. Inception allowed for only a tiny fraction of that variation to be used. The whole point was that the plot within each level of the dreaming had to make sense, had to be designed in order to make sense, and had to proceed from one starting point to one end point. There was no room for intra-dream switching. The ice world could never shift to become a beach; the city Di Caprio and Cotillard had designed could not become a cottage; and the narrative structure within each dream allowed for no alternate solutions. By structuring the adventure as dependent on an architects' creation of a maze, the plot therefore foreclosed what would have been much more interesting: namely, the experience of goal-driven actors in a completely stochastic environment.
And, of course, it meant that the dream could never become a nightmare.