About B/X, my feeling is that the reason the game is so beloved is that, while the rules are certainly not “perfect”, it encapsulates the iconic D&D experience in a perfectly formed package. In just 128 pages you get the foundational D&D content, enough for years of play with no further books: the classic 4 core classes and races; a great selection of the most iconic 100+ spells, 200+ monsters, and 150+ magic items; rules for dungeoneering, wilderness exploration, sea travel, stronghold construction. And what's more, everything is presented in a pretty clean and coherent way, quite unlike the baroque style and organisation of AD&D.