Mystara's rather different from Greyhawk and FR... it's got mostly mono-racial kingdoms, and mostly monoethnic human kingdoms. it also focuses on the law-chaos, rather than the good-evil, axis of the alignment sphere. It also has deities who take an active, personal role. (The king of one of the kingdoms of men happens to actually be an avatar of a major immortal...) The primary area has 4 human countries (a republic — Darokin, two kingdoms — irendi and alphatia —, and a thaumatocracy — Glantri), a wood-elven kingdom, a Dwarven kingdom, and a halfling moot, a smaller reptile-man domain (Malpheggi), an off-shore merfolk city, an the broken lands which are infested with monstrous humanoids - mostly goblins and orcs. And, underneath, the dwarves are facing the Shadow Elves... who are not drow, and are a very interesting counterpoint. It's a setting that has a lot of cultural details already written, and which need little text update, but for which the mechanical handling would showcase mechanical tweaking.
Both Greyhawk and the Realms are strongly laced with mixed race communities; in Glantri, mixed ethnicities are rare, and mostly happen in Darrokin; even then, they're still small ghettoes in the major cities, and some isolated ethnic villages.
It's actually more different than is Krynn (Dragonlance). Dragonlance, the gods meddle a lot from on high; Greyhawk and the Realms, the Gods use clerics to whip up social movements; Mystara, they manifest in mortal form, and go start the changes themselves... without using godly powers other than receiving prayers and mortal manifestation to do so. That Guy following you might be an assassin, or might be Ka the Preserver, watching to see if you're a suitable pawn.
Plus, it's got non-space-worthy flying ships, civilizations in the clouds, tribes of shapeshifters, 6 more human "nations" around the borders, and the Hollow World.
And a rabid fan base that's been quietly ignored by TSR/WotC/HasBro since 1997.