One thing that has really changed over the years is how the young adult book is merchandised. Content has changed somewhat, but not drastically, if the reprints of books like The Vampire Diaries and Thirst are any indication. The categories in bookstores used to separate out "Teen Thrillers" from regular teen titles, and now Barnes & Noble has updated their layout of the teen sections to reflect the modern teen buyer, with a "Teen Fiction," section, a "Teen Paranormal Romance" and a "Teen Fantasy & Adventure" section. And this is how people shop, so it's smart. You can peruse a section containing just the sorts of books you like to read, without having to sort things out as you browse.
The format of books in teen is also drastically different than it was ten years ago. When I was buying teen novels, the only format available was a mass market paperback. Teen books were much shorter then. If an author wanted to write a longer narrative, it was usually written as a trilogy, much like L.J. Smith's The Secret Circle and Dark Visions, or as a series like Sweep. Obviously, teen books are much thicker now, and the packaging has changed with the times, and are released according to the regular adult fiction model. Big releases are released in hardcover first (although some are released as softcover initially, as is the case with some adult fiction titles), then after a certain period of time, are released in a trade paperback format. Rarely are teen books just released in mass market format, and even more rare is a teen title that eventually goes through the first two formats to end up in mass market. The only example I can think of currently is The Twilight Saga, the books of which are released with movie covers in mass market around the time the films come out. No - mass markets are mostly a thing of the past when it comes to teen books, while it used to be a matter of course.