I'm a big fan of Basic/Expert Dungeons and Dragons. From Moldvay to Mentzer to the Rules Cyclopedia - it just hits my gaming sweet spot. It's quick. It's simple. It's easy to modify to suit an individual group's needs. It's this last part that I wanted to address. Because B/X is so easy to modify and gamers and publishers in the OSR often do use the game and its mechanics as a springboard for new options, how many additions and modifications can one have before it is no longer B/X?
Barrel Rider Games built its foundation on adding to B/X through a ton of alternate classes. Rules Cyclopedia provides us with 7 (Fighter, Thief, Magic-User, Cleric, Halfling, Elf, and Dwarf - along with the Druid as a later-level option for Clerics) and BRG produced somewhere around 70 additional classes. Seventy. That's ridiculous. Labyrinth Lord has its Advanced Edition Companion, and there are countless other publishers who continue to introduce optional rules. Not to mention our own house rules that we use at the table.
How long before you're not even playing the same game any more? At what point do you some how move beyond "basic" D&D into something beyond the original scope of the game? One of the things that drives me away from AD&D 2nd Edition is the endless plethora of optional rules, splat books, setting modifiers, countless spells - the list goes on forever. But if I'm playing in a Rules Cyclopedia game and the DM sits down with the RC and is using every optional rule in the book, all the info from the Gazetteer series, and all the info in Poor Wizard's Almanac and the other supplements produced in the line then am I still playing B/X D&D?