A little over 60% of the way through the book and we're only into November featuring the Quidditch match against Slytherin. One forgets how less detailed Rowling is in the early books, the obvious reason of course is the book's initial demographic upon release as well as protagonist's age.
The main "mystery" is coming together, though it feels like a lot of leaping to conclusions to begin with and later added on by more solid circumstantial evidence. However this is coming from a cynic in his 30s probably looking to find faults than a pre-teen who wouldn't think twice and be thinking the same thing.
I've tried to take mental notes of items or people that would prove pivotal in later books, but I'm pretty sure I've gotten wrapped up in the story and missed a few.