More from Supplement I

Additions and Changes
Supplement I, Greyhawk introduces a number of new concepts to the Original Edition of the World's First Role-Playing Game published by TSR. As the first supplement to the popular game, Greyhawk sets the stage for the supplements to come. Named for the Greyhawk Castle campaign run by Original Edition co-author Gary Gygax with Rob Kuntz it is assumed to contain rules, character classes, monsters and treasure drawn from that campaign and demonstrates how each campaign may incorporate additions and changes to give it its own unique tone and feel.
The additions and changes listed in Supplement I, Greyhawk include one new class, Thieves and one new sub-class of fighters, the Paladin. Also included are the following:
Determination of abilities - Supplement I increases the importance of higher ability scores by attaching more advantages to each high score (except wisdom). The original three Little Brown Books have very few bonuses tied to ability scores, a high dexterity +1 to hit with missile weapons, a high constitution +1 hit point per hit die and the charisma bonuses for number of henchmen and loyalty. The advantage of a high ability score in White Box is primarily tied to the primary attribute and earning additional experience.
Alternate hit dice - Supplement I adds the variable hit dice concept to character class. Each character class rolls a different type of die for their hit points under Greyhawk's additional rule (MU & Thieves = d4, Ftr = d8, Clerics = d6). All characters use a d6 for hit dice in White Box.
Variable damage - Each weapon is given a separate damage range determined by rolling one or more dice. Damage is listed for hitting small and medium creatures with a separate column for damaging larger creatures. In White Box, all weapons from (dagger to two handed sword) deal one d6 of damage. Supplement I also includes a to hit adjustment table allowing some weapons advantage or disadvantage in terms of a plus or minus to hit verses each armor type/class. Along with variable weapon damage, Supplement I introduces variable damage for different monsters.
New spells - Supplement I introduces a number of new spells for the existing magic user and cleric lists and extends the magic user spells to include 7th, 8th and 9th level spells. The cleric spell lists  advance to include 6th and 7th level spells.
New monsters and treasures/ new magic items are among the additions found in Supplement I, some of which such as the beholder and the deck of many things have become iconic staples of the brand. Greyhawk ends with a list of tricks and traps for the referee to spring on dungeon delving adventurers.
Depending on your perspective, Supplement I either completes the Original Game rules or changes them. As a huge fan of the original White Box, I tend to pick sparingly from Supplement I for most of my games. The real value of Supplement I in my eyes is the fact that it shows us how the Original Edition can be added to, altered and shaped to give us a game more in keeping with our own personal world vision - tools to tell the stories we want to explore.