I dont really know this setting, I was 6 when they stopped publishing new stuff for it (1996?). Can someone give me a brief idea of why it is interesting? For what I could see it looks like a FR with old-school restrictions (which is cool, I like it), but its way more than that!
Here's some cool stuff and nifty factoids:
* Mystara is a zany, gonzo, "1980s cartoon action hour" D&D setting. (Some Mystaraphiles may cringe at my adjectives, but c'mon...the setting contains Bargle, Warduke, Flamenco Elves, Scottish Highlanders from Earth, Chihuahua Dogfolk, Samurai Catfolk, Magnum P.I., and Ninja Tortles!)
* Mystara is the setting of the Classic D&D game...the Basic/Expert/Companion/Masters/Immortal (BECMI) edition of D&D. There were hundreds of BECMI releases. (Mystara (barely) made the transition to AD&D 2E, but only a few 2E products were produced before the setting was nixed. Boohoo!)
* Mystara is the third (or fourth) oldest published D&D setting, after Greyhawk and Blackmoor (and the Wilderlands of Judge's Guild). The map of (what later became) Mystara first appeared in the Expert Set "blue box". X1 The Isle of Dread was the first adventure set in Mystara. However, B1 Keep on the Borderlands and B2 In Search of the Unknown were retroactively placed in Mystara (though they also exist in Oerth too).
* Mystara wasn't named "Mystara" till later. For much of its product run, it was just called the "Known World". Yet there is one early reference to the planet being called "Urt", but later TSR authors apparently overlooked that. Eventually, DRAGON magazine held a contest to name the planet, but reportedly there were no good submissions. So the product manager just named it himself: Mystara!
* Mystara is a living creature called a Megalith. One of the boxed sets has stats for the planet as a monster! She's got like a million hit points.
* Mystara is the homeworld of Bargle the Infamous! And Aleena the Cleric! (May she rest in peace. Oh wait, later books said she didn't die after all, hooray!)
* The map of Mystara is taken directly from a scientific map of Earth's Jurassic era. (TSR copied it directly from a Time/Life geology book!)
* Law, Neutrality, and Chaos are palpable in Mystara. (But Good and Evil are not). A legacy of the BECMI rules, which only had three alignments, not nine like AD&D.
* There are two main "subsettings" on Mystara: the Known World and the Savage Coast. The Known World is the location of the classic BECMI adventures. The Savage Coast has more of a "New World" feel, with Spanish and Portuguese swashbuckling cultures.
* The Savage Coast setting is afflicted with the Red Curse, which requires the entire populace to continually imbibe an antidote...otherwise they turn into red-colored mutant superheroes. Much of the animal life and people have turned red, and developed aberrant super-powers! (I mean, why bother with the antidote...who wouldn't want to grow a pair of glowing red eye stalks?)
* Mystara is a crazy patchwork quilt of cultures...tiny countries with clearly marked borders. These were portrayed in an iconic Mystaran hex map-style. See: http://mystara.thorfmaps.com
* In Mystara, most (but not all) human cultures (and some of the non-human cultures!) are directly ported from Real World-inspired cultures. For example, in the Known World, there are "re-badged" Bulgarians, Greeks, Romans, Dutch merchants, Vikings, Arabs, Egyptians, Mongols, Scots, Teutonic Knights, North American Indians, Aztecs, and Asian Indians. In the Savage Coast, there are Ottoman Turks, Albanians, Mexicans, Brazilians, Texans, Gauls, Franks, Elizabethan English, Bourbon French, Australian Aborigines, and Amazonian Indians. And many other cultures outside of the Known World and Savage Coast: Bantus, Mesopotamians, Irish, Scythians, and so forth. Tapping real earth cultures gives the DM and players an easy roleplay handle. That's also true of Golarion, and of Forgotten Realms to a lesser, mixed degree, but Mystaran cultures are unashamedly parallels of Earth cultures. And unlike FR and (especially) Golarion, the real world cultures aren't situated in the "expected" place (e.g. Kara-Tur in the Far East of Toril and Tian Xia in the Far East of Golarion) - in Mystara, the cultures are plopped down on the map willy-nilly.
* Some of the "real world" cultures are assigned to non-humans, such as the Elizabethan English catfolk of Bellayne and the French dogfolk of Renardie (mortal enemies of the Bellaynians of course!).
* There are also a few entirely fantastic human cultures - for example, the Alphatian Empire is ruled by a 1000 36th-level magic-users. Yep, a thousand. Alphatia is one of the two "superpowers" of Mystara, along with the Thyatian Empire (inspired by the Byzantine Greek and Roman culture).
* Lupins (dogfolk) and Rakastas (catfolk) are core races, and exist throughout the setting. Each "real world" breed of dog and cat is a different subrace with unique racial abilities. Same for Tortles (turtlefolk).
* There are the usual Elven, Dwarven, and Halfling nations sprinkled on the map. Some unexpected twists: each of the three races has a particular "clan relic" which are attended to by race-specific "prestige classes" (Treekeeper, Forgekeeper, Blackflame Keeper); all (or almost all) Elves are Fighter/Wizards (a legacy of the Classic D&D rules); and there are Flamenco Elves (Spanish-speaking elves)! Half-elves and half-orcs are nearly unknown in Mystara (these weren't PC races in BECMI D&D). Gnomes have a smaller presence than in other D&D worlds (gnomes weren't a core PC race), but there is a nation of Skygnomes who live in a jet-engine-powered flying city, and others live in Giant Robots called Earthshakers!
* There are (probably) no drow. Their niche is filled by the Shadowelves, who are albino in appearance. One branch of them is not (necessarily) evil (but are rather xenophobic and aggressive), while another branch (the Schattenalfen) are evil Aztec-flavored baddies.
* The niche of Duergar is filled by the Moulder Dwarves, who are demon-worshipping craft-wizards (or warlocks!).
* In Mystara are no gods or deities. This niche is filled by Immortals. These (usually) were ordinary mortals who worked their way up to Immortal level.
* In Mystara, PCs are expected to advance to 36th level and then complete a quest to become one of the Immortals....and then keep adventuring! There are two whole boxed sets about playing an Immortal PC...with 36 more levels of Immortal class levels! (Yep, that totals 72 character levels!) It's Silver Surfer "Cosmic"-level play. A few Immortal-level adventures were published.
* There are five "meta-pantheons" of Immortals, called the Five Spheres of the Immortals: Matter, Energy, Time, Thought, and Entropy. (Entropy are the "bad guys".)
* There's a sort of "Justice League"-style headquarters for the Immortals, which is located on the visible moon. It's called the city of Pandius. All five Spheres have representatives there. Umm, even the Entropics have a place at the table.
* There was a series of Creature Crucibles which (like the later Savage Species 3e book) contains rules for monstrous PCs: such as faerie races and lycanthropes.
* There's an Old West-themed country with smokepowder and six-shooters. (The County of Cimarron.)
* There is a crossover with the Lovecraftian author Clark Ashton Smith. Characters from his Averoigne Cycle of stories travelled from France to Mystara and established a realm there. They say they are from "Laterre" (Earth).
* Mystara has its own cosmology, as a legacy of the Classic D&D rules. Officially, the Classic D&D Multiverse exists in a different Reality than the other editions of D&D, which are different Multiverses altogether. A DRAGON magazine article explained that these Realities can only be broached by invoking a Reality Shift, which can only be granted by a high Immortal (of Mystara's Classic D&D Reality) or Greater God (of the other D&D Realities). Nevertheless, a somewhat different version of Mystara does exist in the Great Wheel of the AD&D 2E Reality and (according to the 5E PHB) in the Planes of Existence of the D&D 5E Reality.
* Warduke and the other LJN AD&D action figures are based in Mystara. (Though they also officially exist in the Realm of the D&D Cartoon Show, and in Oerth.)
* The Shadow Over Mystara coin-op video game is considered a classic by arcade aficionados.
* Blackmoor, the personal setting of D&D co-creator Dave Arneson (and thus the oldest campaign setting, along with Greyhawk), was retroactively placed in the ancient past of Mystara. Blackmoor developed a high-tech modern "magitech"-based culture...and the magitech lead to a cataclysm which destroyed the civilization and knocked the planet off its axis. There are a few adventures which bring the PCs to the Age of Blackmoor via a time machine. (Blackmoor exists in other settings too! Some elements of Blackmoor also exist in Greyhawk, though "Greyhawk Blackmoor"'s map is totally different. Blackmoor also officially exists in the Wilderlands campaign setting of Judges Guild, and in Arneson's standalone planetary setting, which AFAIK, hadn't been published yet before he died.)
* There is a sci-fi Galactic Federation in Mystara's outer space which is basically a Star Trek-level civilization. The crash of one of their starships...the F.S.S. Beagle...on ancient Mystara is what lead to Blackmoor's fateful discovery of magitech.
* Mystara has two moons, one of which is invisible. The invisible moon features a nation of Japanese-inspired catfolk and other Asian cultures.
* Bruce Heard, the TSR-era product director of Mystara, in recent years has self-published a blog about Alphatia which provides much more detail about that empire: https://bruce-heard.blogspot.com/search/label/Mystara
* Skyships are a big part of the setting. There was a long running "Voyage of the Princess Ark" fiction series by Bruce Heard, which tells the adventures of an Alphatian skyship crew which explored Mystara (and beyond). Bruce recently asked Hasbro if he could continue to write Princess Ark stories under license...but Hasbro gave him the cold shoulder. https://bruce-heard.blogspot.com/2013/04/OSK.html
Now if Mystara would just be opened to DM's Guild…
* I'm sorry to say that the D&D Movie(s?) were set in the Director's homebrew version of Mystara. It was set in the Alphatian Empire, which was renamed Izmer in the film(s).
* Mystara is the source of a number of standard monsters in 3E, 4E, and 5E, such as the Aranea, Choker, Draeden, Kopru, Brain Collector, Nightshade, Nalfeshnee, Blackball, Roper, and Tortle.
* One of the darklords of Ravenloft is from Mystara: the necromancer Meredoth, originally from the country of Norwold, is the darklord of Nebligtode in the Noctural Sea. Other Mystaran characters have occasionally appeared in other settings such as Planescape: http://pandius.com/mystchar.html
* Here's a Mystara product directory: http://pandius.com/prodlist.html
* Basic/Expert books at D&D Classics: http://www.dmsguild.com/browse.php?filters=0_0_0_0_0_45345_0_0
* directory of later appearances of Mystara in 3E, 4E, and 5E: http://www.pandius.com/myst345e.html
* Mystara Reborn is the main facebook group. Bruce Heard is one of the moderators: https://www.facebook.com/groups/mystara.reborn/?ref=br_rs
* WotC designates the Vaults of Pandius to be the "official Mystara fan site": http://pandius.com
* Last, but not least, Mystara is hollow. There's a whole nother subsetting inside it! The Hollow World. It's a pulp-fiction Tomb Raider-style setting, lit by a red sun floating in the "sky" at Mystara's core. Check out the first ever compiled, georeferenced map of the Hollow World. It's by our extraordinary cartographer Thorfinn Tait, and was just published last week: http://mystara.thorfmaps.com/hollow-world-40/
See, there's plenty of cool stuff for a Known World Adventurer's Guide...