Hmm... I figured out the central mystery of The Da Vinci Code fairly early on, and thought it wasn't particularly well done.

I think a mediocre detective novel can be enjoyed as a puzzle and nothing more - once the puzzle is solved, the book itself becomes obsolete and uninteresting. A good detective novel, otoh, is a good story, and can therefore be reread and new things will make themselves known upon the rereading, not just the clues one may have missed before, but subtle character interactions that may or may not be clues, but which lend the story depth and texture. And also, I find with detective series - like Nero Wolfe or Amelia Peabody - after a while, the mysteries become secondary to loving the characters and wanting to visit with them.

I do think Rowling sees herself as writing a mystery - hence her quoting of Dorothy Sayers in interviews - and so she sees as less important things that make a fandom excited. Of course, Rowling also stacks the deck against the reader figuring things out because of the way the story is serialized. And also the way she pokes fandom and watches it jump.

I don't think I have anything useful to add, so I'll just shut up now.