"So then what videogame designers really do is create digital worlds that invite play.
What does this theory buy game designers, or anyone else in the community for that matter? For starters, it allows us to see many of the things that folks from Callois to Crawford, Koster to Costikyan, and Bogost to Juul have variously considered essential features of videogames—things like uncertainty, conflict, fun, competition, and goals—as part of a palette of strategies for luring gamers into playing in their worlds rather than simply manipulating them."
The quotation above is from the article A Theory of Games For Just About Everyone. It challenges the opposition between free-play and gameplay, also regarding play as a mental state, rather than an activity.
My personal, perhaps unrelated thoughts on the subject:
(via Raph Koster)