Mystara is a "kitchen-sink" world, with a number of nations described by the Gazetteer series (for example "The Gran Duchy of Karameikos" corresponding to medieval Bohemia). Most often than not, each kingdom has a good strong ruler, but also a number of villains. In the case of Karameikos, the good Duke Stephen is opposed by his evil cousin, Baron Ludwig von Hendricks the Black Eagle, with his court mage Bargle the Infamous. Many problems are entrenched in the politics of the kingdom. In fact, the BECMI edition of the game supported high level play, so that Basic PCs (Level 1-3) started their career in dungeons or on local quests; Expert PCs (4-14) explored the wilderness, facing regional threats, and met important political figures; Companion PCs (15-25) became rulers, carving and menaging their own domains, interacting with other political leaders and waging wars; Master PCs (26-36) were leaders of nations, but they were so powerful that they started their own quest to ascend to Immortality (godhood). So, in the Expert and Companion phases of the game, politics became very important. You could meet the personalities of your kingdom (or of other nations), help or oppose them, and change the destiny of nations. The political nature of the setting is evident in many modules, for example X10 Red Arrow, Black Shield (a sort of World War in which the PCs, as ambassadors, must travel to many nations and forge an alliance against the Master of the Desert Nomads) or CM1 Test of the Warlords.
Many BECMI adventures set in Mystara became unforgettable classics: Keep on the Borderlands, Rahasia, The Lost City, Night's Dark Terror, Isle of Dread, Castle Amber, Master of the Desert Nomads... Plus many, many hidden gems.
The Gazetteer series is an high point in the history of the game, with such masterpieces as The Grand Duchy of Karameikos, the Principalities of Glantri, and The Shadow Elves.
Besides, the world was hollow, and you could travel to the centre of earth where lost civilizations dwelled...
Plus, there were many different over-arching metaplots: the hidden magical source known as the Radiance; the recurring machinations of the evil Immortals Thanatos, Alphaks, and Atzanteotl; the eternal Quest for the Blue Knife of the orcish race... It was really a rich and detailed world, on the same scale as Greyhawk and almost as the Realms.