I think that it is important to note the Melee/Wizard/TFT system, like Chainmail/OD&D, spring from the wargaming roots of the hobby. Melee itslef is a wargame which can be used to adjudicate combat in RPG's--but it does not (beyond including fantasy creature archetypes) present itself as an RPG. Melee never discuses creating characters nor adventuring, only creating figures and combat.
Instead of the D&D way of randomly generating 6 attributes, Melee builds figures of 2 attributes using points the player can add to a base of 8. Your 2 attributes (Strength/Dexterity) determine weapons your figure can use, how much damage it can take and how likely it is to hit someone in combat. All combat, damage and other uncertainties are resolved using six sided dice.
Armor does not make you harder to hit. Instead the heavier the armor, the slower you move and the harder it is for you to hit others! The benefit of armor is that it absorbs damage taken. So lightly armored characters move faster and act sooner...but heavier armored figures are harder to kill. It is a unique give and take system and quite different from the D&D combat model.
As a wargame, Melee delves into combat details and modifiers for engaged vs disengaged figures, dodging and defending, figure facings, (rear flank, etc.) creature sizes as represented by counters.
Map and Counters
To play Melee (and Wizard or TFT) a map and counters were required. All Metagaming adventures (except Tolenkar's Lair) came with map and counters. The TFT system required use of Melee/Wizard. The hex map and counter use goes back to the wargaming roots of the system. It is interesting to note that as D&D moved away from its wargaming roots, Melee/Wizard/TFT retained its wargame roots.
Why it worked
It was SIMPLE: simple to explain, simple to make a figure, simple to adjudicate, and simple to play. My hope is the foll HOW RPG will retain this simple logical elements while straying towards RPG use rather than further codifying a fantasy wargame experience.