I only ever got to play a few sessions in Mystara decades ago, and mostly know only bits and fragments of it from what I gathered from friends and glancing at their books at the time, as well as a few things I read online later on. I was intrigued by its cosmology and immense variety of cultures, and the idea of a Hollow World that had another world inside its main, primary Known World (following the concept of a Hollow Earth), with the inhabitants of each largely ignorant of the other. But occasionally, someone from the surface "known" world of Mystara would fall through the cracks to land in the mysterious, unknown world underneath, which was lit from a light that radiated from inside of it (IIRC).
The world had a strange mix of many cultures and pretty much every type of classic fantasy creature or mythical version of real world culture had a place or analog within it. I never ran into much material for it, since I got into the hobby in 1990 and didn't really start buying books till 1991, by which time AD&D 2e was out (which I preferred over Basic and quickly made the change), and I believe Mystara may have already been cancelled by then. But it was the one thing about Basic that always stuck with me due to its strange cosmology and myriad lands, cultures and peoples.