If you had to boil down the reasons to play Mystara/Known World instead of any other D&D setting and present it to someone fairly new to the game (say, a few months' experience) and present it in a list of 3 things, what would you say these are?
What makes Mystara different than other settings? What does it offer that you can't get in other official settings in the D&D pantheon? What can you do with it? What makes it better at those things? What are the "selling points" of Mystara?
1. Mystara provides an extremely rich and beautiful tapestry of a setting including its geography, history, races, people, languages, religions, cultures and personalities which when applied within a campaign makes it fairly distinct from other D&D settings.
2. Besides becoming a knight/noble and/or landowner, Mystara allows one to explore setting-specific goals: (1) Becoming a GrandMaster of one of the Secret Schools of Magic in Glantri (2) Attaining Immortality with the assistance of an Immortal willing to sponsor you (3) Rising to the caste of Merchant Prince of Minrothad (4) Realising the Dream of Al-Kalim for Ylari (5) Finding the Blue Knife and uniting the Humanoid Tribes of the Known World (6+) so many more...
3. The F.S.S. Beagle, The Nucleus of Spheres, The Radiance, Old Ones, The Immortals, Radiance Receptacles, Black Flame, Wrath of the Immortals, the War Machine, Merchant/Bargaining Rules, Rules for Naval Battles, Humanoid Campaign, Lycanthopic Characters (Night Howlers)...etc
4. Expansions of the Known World - Undersea (the life beneath the Sea of Dread), Red Steel/Savage Coast, Serraine (a flying Gnomish City), Hollow World and even the fan based project for Mystara 2300 BC
And the support structure is still there even after it was dropped as a setting around 20 years ago (the below are all free):
Vaults of Pandius - an online website which is updated regularly and maintained by fans across the world
Threshold Magazine - an ongoing collection of articles expanding on the mythos of Mystara
thePiazza.org.uk - a forum such as Enworld where fans of Mystara congregate and can share ideas
GazF series - 10 fan-made gazetteers with maps on additional areas in the World of Mystara, and these then do not include fan-made gazetteers on smaller areas like the Mini Gaz1 The Central Altan Tepes
Codex Immortalis I & II - Fan-made gazetteers on everything you would need to know of the religions & cults and the Immortals of Mystara.
Maps - thanks to the efforts of Thofinn Tait one can have top quality maps of the Known World and beyond.
Newbie Guide to Mystara - fan-made summary of all the Gazetteers.