I've been thinking about this too. Fanny is not a doer in the story. She doesn't make things happen. Things happen to her. She blossoms some, but she is the constant while all the bustle goes on around her. I think it's surprising (or was on my first read) that Austen makes this meek shy girl her heroine.

While Fanny is not perfect she seems to have the best judgment of all the characters in the story. We get impressions through the Fanny filter of others' behavior.

Anyway I think it's wonderful and that's why I love this story too.