You can (well, could... past tense) formulate theories! The story was so intricate and amazingly well done... it simply has so much in it and so much appeal to everyone that it was bound to be a hit. [Random thought: Do you think that it would have been as successful if a girl had been the protagonist? Of course not. You just lost about half of your target audience right there.]
I liked how there was always more to the characters than just what was on the surface. Snape, Draco, Harry, Dumbledore, Sirius, even Ron. You could read into everything! And you get to jump into a world with magic! Ah. I love Harry Potter. I just love the adventure, and the purpose. Just... everything. It's so hard to explain.