My least favorite Austen book, Northanger Abbey's greatest weakness isn't a lack of deftness by the author but Austen's success at rendering a flawed protagonist. Our "heroine" is Catherine Morland, a painfully naïve teenager making her first long trip away from home. She's a bit dim, our Catherine, and so eager for "attachment" that she is easily swayed by the character of her companions.
In the first half of the novel, Austen is rather brutal in conveying Catherine's flaws via a distinctive series of parenthetical asides and wry juxtapositions with more shrewd characters. In comparison with her more famous works, Northanger Abbey is very nearly a straight satire. While Austen has always included a rebuke of societal trends and values she found abhorrent (see the vapid vanity of the father and sister in Persuasion), her protagonists are usually incredibly compelling and likeable despite their flaws.
Austen departs from that formula here in order to more deliciously attack the superficiality and scheming inherent in that era's society. The reader is much more likely to sympathize with Catherine's love interest in Northanger Abbey, a sarcastic yet kind Henry Tilney. But Henry remains a distant and somewhat vague personality throughout the book, and we are chained to Catherine's feelings and thoughts, regardless of our opinions or preferences.
The most enjoyable sections of the novel, for me, were when Austen let loose in depicting Catherine's new companions as terrible and annoying in contrast to Catherine's good opinion of them. I also found the clear undercurrent of Austen's frustration fascinating. As a woman, there was a clear delineation between what kinds of novels she could produce and what was considered more serious efforts by the men in her profession.
The second half of the book, and descent into commentary on Gothic literature, is where I grew disinterested. This seems to be an unpopular opinion. Though some of my former English teachers might be terribly disappointed in me, I had to slog through much of the Abbey descriptions and finished the book by momentum rather than curiosity.

However, I do think the book is worth reading as a contrast to her other works. I recommend Northanger Abbey to those who dismiss her work as Chick Lit, and even more forcefully to those fans that recognize only the romance and dreaminess of Mr. Darcy.