From Greg Costikyan's weblog (commenting on the state of the industry): "Electroplankton, on the other hand, I'm very skeptical about; a toy with no evident goals, and no goals means "pointless."
That sentence has actually generated a heated discussion on the comments forum, where Costikan made a good point:"Games have goals.. not necessarily explicit ones. Interaction without an objective is meaningless."
It might be true, but I am not sure that is the case for Elektroplankton, as your goal as player may be get better as a composer / mixer your tunes. Nintendogs - which could also be seen as a non-game - has a much clearer goal (the survival / happiness / success of your puppies).
So maybe the biggest challenge in nongames design is giving the player the opportunity (and incentive) to create their own goals and see the original 'implicit goals' as the means to achieve them.