Book Musings: Adult vs YA - what's the difference?

I'm reading a book right now that is sold as a YA fantasy. I'm pretty sure the reason it's shelved in the YA section is because the protagonist is 16. But the farther I get into the book, the more I'm convinced that it should be marketed to an adult audience. There is nothing remotely "teen" about the book.

That got me to thinking. Besides the age of protagonists, what is the difference between YA and Adult reading? Here's what I came up with.

1. Themes and situations. The themes found in Adult vs YA are a vastly different.

Some example of themes I've found in recent adult books I've read: Dating with kids. Sex. (I know there are YA books out there with sex, but the majority of the YA I read has little to no sex.) Work-life-balance. The joys and struggles of maintaining a long term relationship. Parent-child relationships from the parent's POV. These are all themes that would not pop up in YA books. Their ones I'm personally interested in because they more reflect my current stage of life.

Example of YA themes I've seen: Conflicts with friends. Love angst. Friendship angst. School conflicts. Parent conflicts (from the teen's POV). Coming of age.

2. Pacing. I think adult novels often have a slower pace. Not always, but I defnitely think authors can get away with a methodical pace in an adult novel. YA, on the other hand, tends to be very fast-paced -- one of the things I personally love about it.

3. Length. It's not very often I see a 700 page YA novel. I see them quite a bit in the adult section. I think long YA books are often sliced into multiple novels.

4. Age of the Protagonists. This is sometimes a good indicator of where a book should be shelved, but not always. One example I can think of is Mistborn by Brandon Sanderson. One of the main characters is sixteen at the start of the series, but the series is definitely written for adults. The characters, even the young ones, are all very adult with adult problem. So I don't think protagonist age should be the only thing taken into consideration when calculating the target audience for a novel.

What do you guys think? I'd love to hear what you guys have to say about YA vs Adult, and how you categorize the two.