White Box Class

Character Class in OD&D
In Men & Magic the player is given the choice of one of three classes for the player character, fighting man, magic user or cleric. The fighting man class is the basic warrior, knight, mercenary, swinger of swords from myth, literature and legend.  The magic user is the basic caster of magic spells, sorcerer, wizard, and magician. The cleric is a sort of hybrid between the two with a religious theme. The cleric can cast magic, but of a different kind from the magic user. The cleric can fight better than the magic user, but obviously not as well as the fighting man. Unless we turn to the supplements, that is it for class in the white box. I have played many sessions using just these three classes and find players very capable of individualizing each character through play without the need for complex class mechanics to do so.
OD&D (and later D&D editions) is a class based roleplaying system. It uses the idea of distinct classes to organize and control the abilities of the characters. The goal is that each class give a play experience distinct from the other classes. The iconic three class system makes sure that there are distinctions between player characters of different classes and incorporates progression in relative power called levels. Characters start out at low level and as they acquire experience advance in levels and become more powerful in doing so. This is all tied to the class system in D&D.
Many of the early roleplaying games were also organized along the lines of character classes, although some games called them career paths or professions, etc. I suspect this was often the case of imitation of the world's first fantasy roleplaying game. The second published fantasy roleplaying game, Tunnels & Trolls, uses a similar class system of warrior, wizard and rogue.  The rogue, like the cleric in OD&D, is a hybrid character able to fight and cast spells, but at the cost of being less competent at both than the "pure" classes. The Fantasy Trip, Melee and Wizard, did away with the hybrid and used just a warrior or wizard two-class system.
I think there is much merit to the original three class system, although I have played many class-less systems and have even devised a variant of white box that only has one class: adventurer. By dividing the player characters into magic users and fighting men, white box D&D mimics the usual division found among its sources in fantastic literature, myth and legend. Iconic characters are often great warriors or powerful magic users, occasionally both. The inclusion of the cleric makes a significant statement about the intention of the white box game, I believe. I cannot speak to the intent of the designers, but character class availability is one way to shape the game by calling out who the principal characters will be. In white box,  fighting men, magic users and clerics will shape the action. By implication of the cleric class, religion matters and divine power exists to shape events.
Spplement I, Greyhawk added a couple new classes to the milieu, paladin and thief.  The paladin is a fighting man totally devoted to Law and as such is a subclass of the fighting man class. As the game progresses through it's many editions, the inclusion of new classes and subclasses will dramatically change the cast of characters shaping in-game action.
The thief is a game changer in many ways and has generated some controversy as to whether the change is positive or negative. The thief is relatively weak in combat, being limited to d4 (like magic users) for hit points in the new variable hit die system of Greyhawk, and limited to leather armor. The thief class's chief distinction as a class comes in the form of percentage based "thief" skills. At low level the skill percentages to succeed are modest. The fact that the skill list includes mostly abilities that I (and presumably others) as referee frequently allow any character to attempt using "house rules" under the white box, skills such as hide in shadows, move silently, open locks, look for traps, listen at doors, brings into question whether these abilities should be the sole province of the thief class. Since the thief starts out with very modest percentiles in these skills and we can assume these are skills in which the thief should be better than other classes, it can pose a dilemma.
Men & Magic makes an attempt to offset the advantages found in player character non-human races with class restrictions and level limits. Humans are the only race that are unlimited with respect to choice of class and level advancement. I like this as I tend to prefer to referee a "humanocentric" (human dominated) setting. I find doing so allows the non-humans to be more non-human (not just humans with beards or pointy-ears). The thief class is open to the non-humans as an unlimited class, which seems to me to go against the original logic. As a player and referee I have used the thief class as presented in Supplement I for many years, mostly to my enjoyment and the enjoyment of others.  I have also experimented with custom thieves borrowed from other sources and designed my own version based on what I prefer to see in the thief class. It is this latter interpretation that I most prefer these days. I should probably say more about that in a future post.
Supplement I brought many changes to the white box when it was released in 1975. Together with Empire of the Petal Throne, released the same year, TSR gives us two fine examples of putting the LBBs through their paces in terms of campaign design and game play. Looking beyond the rule additions, new monsters and magic, Greyhawk and EPT each teaches us how to adapt the white box rules, alter them to suit a unique vision of the game, and to use our do-it-yourself skills to take our fantasy roleplaying into the beyond.