Witty, Clear, Layered
You might be wondering how those three items go to together. It's a bit of a strange mix, but Austen pulls it off. Northanger Abbey is an undoubtedly witty book. The dialogue and the narration are very clever and clear. Stylistically, Austen is mastering the art of brevity here. She's a pro at using succinct phrases and quick observations to elicit a laugh. She's also a pro at letting her characters' dialogue do the heavy lifting of garnering laughs. Each character has their own unique speaking style, from John's blustering exclamations to Catherine's tumbling phrases, to Henry's endless wit, to Isabella's rapid-fire chatter. The narrator's style, witty, clear, and often succinct, rises above her character's often chaotic exchanges, bringing everything together.
So we've got witty and clear covered. Now, for the layers. Northanger Abbey has a lot going on in terms of plot and themes: it's a love story, it's a comedy, it's a tale of friendship, it's a tale of growing up, it's a parody. The style reflects these plot layers as well. Or styles plural, rather.
The narrator often uses different styles to narrate the plot and to directly address the reader. When narrating the plot, the narrator's style is generally clear, witty, and relatively concise.
For example, when describing John Thorpe's reunion with his sisters, the narrator's style is fairly concise and to the point, with a brief humorous clause tossed in for a laugh:
On his two younger sisters he then bestowed an equal portion of his fraternal tenderness, for her asked each of them how they did, and observed that they both looked very ugly. (7.43)
When indulging in asides, the narrator often adopts a more rambling and overtly mocking style. This more rambling and wordy style also crops up in instances where the narrator is spoofing the Gothic genre and directly commenting on the links between typical Gothic novel plots and her own. These asides are making fun of the styles of Gothic novels and moralizing tracts that were popular in Austen's era. These tracts were really pedantic, which means that they showed-off by using big words and long sentences. While parodying social commentators who lectured on the "proper" behavior for young ladies, the narrator uses lengthy phrases with lots of clauses. One sentence goes on for nine lines in the last paragraph of Chapter 2. During one aside on the virtues of novels, the narrator again uses lengthy sentences and numerous clauses that are only occasionally broken up by a shorter sentence or phrase. This goes on for almost two pages.
Sentence length, the number of clauses, and use or lack of narrative direct address are all key indicators of what the narrator hopes to accomplish in any given passage, be it a humorous account of events, a satirical rant aimed at the reader, or a parodic rendition of a Gothic text.