Thanks for all the insights. It's a fair point that opposites can attract. (Although I for one don't see Mr. Knightley/Emma as one of those pairings on a reserved/open basis -- I don't see Mr. Knightley as all that reserved, except for his lack of interest in dancing. He socializes with nearly everyone -- the Westons, the Bates, Mr. Woodhouse, etc -- and is always out & about.)

I just wish Ms. Austen had planted some inkling of a shared interest somewhere, or something that Jane might admire in Frank. The closest I can get is that Frank Churchill did sing to Jane's music in Chapter 26. But it's not very much. In a book where there are hints about almost everything once you know the story, I just wish there as a bit more to make Jane's attraction to Frank seem reasonable.