I just got done "reading" Raph Koster's Theory of Fun.  I say "reading" because his presentation is unique in that he combines cartoons with text to get his message across.  And his message is unique.  He starts out by pointing to the fact that games are really about looking for and recognizing patterns.  When we see no pattern, also referred to as noise, gamers get frustrated and stop playing.  The game gets boring once a player has mastered the pattern.  Thus fun is somewhere in the middle between noise and mastery:

Noise (no pattern) { Fun { Mastery of Pattern

         Noise   = Frustration

Mastery   = Boredom 

      thus...

          Frustration { Fun } Boredom

A conclusion that we can draw from this is that games that the player can never quite master is the most fun that one can have.  All the great games according to this theory have lasted and survived because they have an infinite amount of possible ways that the patterns can be applied.  Some examples would be chess, chinese checkers, go, backgammon, etc.  These all have basic patterns of play, but endless possibilities as to how to use those patterns.

One caution, however, is that this theory applies to each individual.  What I mean is that each individual has a range of fun based on their ability to perceive and recognize the pattern of play. The range of fun for one player will not necessarily be the same range of fun for another player.  Thus, when coming up with a game that will be fun for the most amount of people, your pattern of game play needs to be near the middle of the continuum. In other words:

Noise > Most amount of players having fun <>

Thus, a game is limited by the highest amount of noise that its players can tolerate and the lowest amount of boredom. Not so complex that the player can't easily pick up the game, but not so simple that it is easily mastered.    

To summarize, the more possibilities that there are in applying the pattern of play, the more fun the game is.  In order for a game to be fun for the most amount of people, however, it needs to have an easy pattern of play, but a wide variety of possible ways of applying that pattern.  

I recently discovered geocaching.  For those who don't know geocaching is a form of treasure hunt.  Participants use some form of GPS to locate "caches" which are the treasure they are seeking.  A typical cache consists of a waterproof container, a log book, and some type of treasure.  The name was originally gpsstashing, but it was suggested that stash may have negative connotations so the name was changed to geocaching.  There are a number of sites that are dedicated to the pastime and there are caches located in over 100 countries and on all seven continents.  The first geocache was placed on May 3rd, 2008 and it was first found and logged on May 6th, 2008.