Types of Play, an article by Evan Robinson published at Chris Crawford's Erasmatazz proposes a new classification system for computer entertainment systems.
Among the definitions presented by Robinson, the one that better applies to nongames would be "Unstructured Play"
Unstructured Play: Interaction with a system in which the primary goal of the user(s) is examination of the system's behavior. Also called 'Exploration'.
This definition indicates an interesting characteristic of the genre.
Even taking in consideration that, as put by Robinson, "one user's Toy is another user's Puzzle and yet another user's Game", it is fair to say that this kind of Unstructured Play is often meant to be a sort of toy. Toys offer a kind of freeform play often refered to as "Paidea" (as in Frasca's MA thesis), where there is not a 'win' or 'lose' situation (which would be present in games better defined by the term "ludus".
"Ludus", then, suggests a set of defined rules and game mechanics that don't give the player the impression of being manipulated arbitrarily by the system with no palpable explanation inside the game rules. In "paidea" (and therefore in toys, nongames and unstructured play in general), the absence of the system as a foe might liberate it to surprise the player more often and unexpectedly - something even desireble when exploring the system's behavior.
The "interesting characteristic" is that, since there is no struggle for control of the game state and outcomes between the player and the system, the player might as well abdicate his "mastering" of the system more often during play.