This is something that is mentioned often enough - that Birthright is enjoyed by many because of its flavor - it is simply "cooler" than most worlds. Why is that? From where does BR draw its inspiration?

What are the defining characteristics of Birthright? Here is some of what I feel gives it flavor (doubtless, I forget something):
-The magical feeling of the setting. Cerilia is a magical land - more magical than most settings. Why? Because magic feels more magical here - it is less the "+1 longswords come 13-a-dozen" and more the "this was my father's sword, with which he slew the rebellious baron of Ghellaen." Wizards and sorcerers are very rare and subtle - they can wield great power, but most of all, they are all mysterious - and a mystery is a power unto itself.
-There are truly mythological monsters for characters to face, rather than the "gorgon-a-gogo" of most other D&D worlds. Each legendary enemy has a story of its own and wields terrible power. This also contributes to the magical feeling of the world.
-The historical feel - the continent of Cerilia has a history, not too detailed, like some worlds, but rather a feeling of history - history has a meaning here, because things used to be different.
-The gods. The gods aren't omnipotent here - they are very powerful, yes. But, like in greek or norse myth, a sufficiently powerful mortal can still challenge even the gods themselves.
-The races - elves, dwarves, halflings - have more character of their own than in other worlds. Of course, they are what you make of them, but they are generally more different from humans than elsewhere. And the subcultures of humans are all different in very noticeable ways as well - and it is easy to relate to the differences, because they have real world counterparts.
-The politics of the land - characters can be regents and make a difference in that regard.

All in all, there is more of an epic feel to Birthright than any other campaign I have seen published by TSR/WoTC. Why is that? It is because everything the PCs do is significant. Here, the PCs have a real impact on everything, potentially from 1st level and up. And you don't just defeat _a_ dragon - you defeat one of a very few remaining dragons, a legendary beast unto itself.

Then, what are the primary sources for inspiration in Birthright?

-Tolkien. Of all TSR worlds, Birthright is perhaps closest to Tolkien's works - you can draw some parallells, that while not entirely correct, still has some credence - the Gorgon (Sauron); Azrai (Morgoth); Kal-Saitharak (Barad-dûr); the Anuirean Empire (Gondor); Wizards - rare and mysterious; elves - distant from men. No such comparison is entirely accurate, yet there is certainly similarity.

-Arthurian myths. The king of the land; knighthood, chivalry, uniting a shattered land. In the Ruins of Empire, Mhoried always struck me as having an "Arthurian" quality - perhaps that is just the picture of the Mhor, though. From Arthurian myths, you can also draw wizards of great power, mystery and rarity (Merlin, Morgan le Fay), faeries, and the divine right to rule.

Doubtless, there are many other sources for inspiration - these are the ones that I see that lie closest to the "core" of BR, though - Anuire.