1) Readers WANT to suspend disbelief.
How likely is it that Muggles would never, ever notice magical people living among them, dressing differently, talking differently, and often completely ignorant of incidentals like telephones, subways, and electricity? Readers can swallow anything given enough reason, and more importantly, they want to swallow the premise and be transported to another world. I don’t have to follow history or convention to make a fantasy world viable. My fantasy can have newspapers, books, solar heating, and indoor plumbing, yet no magic, gunpowder, or explosives.
2) Anyone can be a hero.
An eleven-year-old orphan who wears glasses and had a horrible childhood can still turn out to be a good person and even a leader. People never tire of rooting for the underdog and people will overlook flaws when they find the good. My hero owns slaves. My heroine IS a slave.
3) People love to hate the bad guy.
Villains cannot be TOO evil. Fantasy cries out for really evil antagonists. No one whines about Voldemort not being nice enough.
What did you learn from Harry Potter?