Fighting Man, Magic User, Cleric

The Three Original Classes
White Box introduced the world to a new pastime and in many ways defines the resulting hobby today. Mr. Gygax and Mr. Arneson re-imagined the stories they loved, involving heroics, magic and the supernatural, and invented a new type of game, one where players get to experience through an alter ego, the player's character, those adventures read about in mythology and fantastic fiction and seen portrayed on the screen and in comics. Archetypes and icons figure prominently in the design of their game including the Fighting Man - warrior, knight or strong-man, the Magic User - wizard, sorcerer or wise-man, and the Cleric - crusader, priest or holy-man - Heroes all, who conquer adversity by using their bodies, their minds or their faith. The inclusion of the Cleric, inspired by the medieval warrior-monks of various religious orders such as the Knights Templar, I think says much about the game's creators, their interests and their intent for the game's default setting or milieu.
The class system seems a way to organize, individualize and balance player character (PC) abilities and specializations, giving each PC a unique role to play in the party mix. Combined with Tolkien-esque races, the White Box player can choose among a number of unique characters. Backgrounds and fully fleshed PC personalities are not encouraged in the rules as written, partially I believe because of the high anticipated mortality rate of low-level PCs. PC personality is what develops through play and the longer the PC lives and adventures, the more personality becomes established.
White Box essentially encourages each referee to make the game their own. This often involves modification of some aspect of the rules including the addition of alternate classes and sub-classes of playable characters, frequently a part of campaign development as evidenced by the official supplements. My initial discovery of White Box included a near mindless desire to play with all the options without regard to source or effect on the nature of the milieu. At this point I am more deliberate in my choices and ask myself whether the original three classes aren't enough. How is the campaign shaped by offering additional PC classes?
Supplement I introduces the thief class and paladin subclass. They can be viewed as specific to the Greyhawk campaign along with higher level magic spells and additional monsters and magic items that all help shape the particular milieu of Greyhawk. Most players, myself included, have treated Supplement I as additional material which we add to the White Box without much thought - an "upgrade". I think it is more than that, however. It is also our first model for how to customize White Box for a specific campaign. It is the rules for playing White Box in Greyhawk.
The White Box gives us the basics and with the Fighting Man, Magic User and Cleric many fun and exciting games can be played. Adding additional material should be done deliberately and with purpose because it will change the game to some extent. One of the reasons I keep returning to the original game is White Box encourages making the game your own.