After spending most of last class discussing the difference between work and play, I don't think there is any definition of play that can be applied across all situations. While Huizinga states that play takes place in a separate world (which he terms the "play-world"), this can only be partly true, particularly in the realm of professional sports. Professional sports are definitely "play" for the observer, but as the occupation of professional athlete has become more desirable, the competition to become one has become so intense that athletes put more work into their jobs than most other people. Also relevant to this point is the fact that athletes do play a game for a living, so in order to differentiate between work and play, one must look beyond enjoyment as a criteria, because it is also possible to enjoy one's job. Therefore, to me, play must have a carefree element, whether it is structured like sports or unstructured like the play of a child. Overall, I do not think there can be a unifying theory of play though.
Determining the difference between work and play is becoming exponentially more difficult in the digital age. Technology is blurring the line between the two, and I, for one, am undoubtedly opposed to it. There is a movement that is gaining momentum known as "gamification" that is the application of game play to non-game situations to create more interest and motivation, which can be seen in new applications such as Foursquare and MyTown, in which you gain points and even titles for checking in at different locations. Gamification is even being used in corporate settings in an attempt to motivate employees. One of the most popular uses of gamification recently is the grading system employed by Lee Sheldon at the University of Indiana. The grading system made students earn points in order to "level up" to better grades (an overview of the syllabus can be seen here: http://terranova.blogs.com/terra_nova/2010/03/build-your-own-sheldon-syllabus.html).
As a psychology major who is very interested in motivation and goal-oriented processes, I see many problems with this surge of gamification. First of all, gamification creates an unrealistic mindset (i.e. "if I do X, I will be rewarded with Y"), whereas real-life interactions are far more variable than the goals and achievements of video games that gamification applies. Secondly, gamification provides extrinsic motivation (external reward), which can interfere with intrinsic motivation (self-motivation, i.e. done out of personal enjoyment) and lessen motivation overall. This mainly applies to the use of gamification in the workplace and the classroom, in my opinion. While tangible rewards may be effective at young ages in schools, students should have developed a sense of drive and direction to their lives as they mature. A grading scale such as Sheldon's at a university oversimplifies life down to a system of points and experience and leveling up, rather than encouraging critical thought that comes from the students' interest in the subject. This is where the difference between work and play is necessary. As we grow up, we are given more and more responsibilities, so we are not able to play like children anymore, but this fact makes it even more important that we find time to play games. To me, the separation of work and play is second only to the separation of Church and State. We should be taking breaks from the stress of life by playing games, not combining the two.