- are not going to have graphic sex and/or foul language in the romance novels.
- are shorter, less complicated, and easier to read.
- are about the struggle between good and evil with big themes that are easily understood.
- Graphic sex in YA books has been around a long time. Forever, by Judy Blume was first published in 1975.
- Many YA novels are not shorter and easier to read than adult novels. The Harry Potter books, Jonathan Stroud's Bartimaues series, and The Book Thief, to name a few, are longer and more sophisticated than many adult novels.
- It's not true that YA books are about big struggles between good and evil, either. An awful lot of them are vapid stories about shallow girls fighting and having sex.
Dystopian is enjoying huge crossover appeal. I think this is the one genre about which it can be said that YA books are more simplistic than their adult counterparts. Teen dystopians are fast-paced, mostly written in first-person and often in the present tense. They are about action, not about characterization, and most don't offer deep, thoughtful commentary on the state of the world. They are full of cartoonish government bullies and kick-ass heroines. I think adult dystopians are deeper and require more thought.