Better editing: Adults read to escape while kids read to learn. Even if it is unintentional on their part, every time a child picks up a book, they're expanding their vocabulary, improving critical thinking and discovering new concepts at a much higher rate than adults do when they read, which means editors have to work twice as hard to make sure kids don't find typos, plot holes and faulty logic.
Faster pacing: Shorter attention span, lower comprehension levels and early bed times mean kids don't spend as much time reading in one sitting like adults do. Children's lit is usually much shorter, which forces writing to be more concise and conflict resolution to happen faster.
Stronger female leads: Females not used as sex symbols are an anomaly in adult fantasy, yet they are the driving force in teen fantasy. Only as the demographic shifts to more male readers as the audience gets older do you find fewer female leads and more females as mysterious soothsayers, damsels in distress and evil queens.
Character first, setting second: Maybe it's because of the word-count restriction thing, but children's authors spend less time establish worlds and rules and more time building characters. In order for a young reader to connect with and keep reading a book, they have to relate to the main character. Adults, however, tend to read a book even with a despicable main character if they find the concept interesting.
Truer to motive: While there are always exceptions, children's fantasy isn't trying to make a political point or driven by societal morals like so much adult fantasy is. Of course they are exceptions to this (His Dark Materials by Philip Pullman comes to mind), but political commentary tends to go over kids' heads or even turn off parents and librarians who are the primary book buyers.
A for a special Friday Five bonus...
Crossover authors: Don't be fooled. While many best-selling adult authors are amazing writers, they can't always pull of younger characters and vice-versa. Here are five authors who cross the age divide, some more successfully than others. But I'll leave it up to you to decide who does it best.
Maria V. Snyder
Orson Scott Card