Mostly the plots. I haven't read Mansfield Park which is supposed to be the most atypical, but her other books are ordinary plot-wise. It's probably partially that she's one of the earliest writers who did "modern" chicklit/romantic comedy or however you want to call it, so she does not come off as very innovative after 200 years. Her satire of 18th century sentimental novels and late 18th/early 19th century gothic romances is almost completely lost to 99% of her readers, since those genres have aged much worse than Austen and reading them to get the context would require a lot of effort from almost anyone. (I'm sure there's people who read Mysteries of Udolpho for pleasure and I thin Valancourt Books is doing pretty okay with its reissues of forgotten gothic novels including the ones referenced in Northanger Abbey, but those groups are vastly outnumbered by Austen's readers.)
The characters are just as deep as the reader wants them to be, which I think is one of Austen's strengths and what has made her endure for so long. I maintain that reading Pride and Prejudice as a story of two shitty people whose faults make them just as compatible with each other as their supposed virtues is more interesting than reading it as story of two straightforward romance protagonists, but the story works and is enjoyable either way. Everything I know about Austen suggests that she was smart enough and cared about her writing enough to create that effect intentionally, but it means that what the reader puts into the book is what they get from it. You can read Austen for deep characters and you'll find them, but if you don't, you're still able to enjoy the books.