1. The author doesn't read, isn't informed what today's children and young adults read.
2. The author doesn't know whose story to tell and skips from one character to the next.
3. Unfinished stories (that stop when the author loses interest in continuing writing) and loose ends (in the worst case, a missing kitten remained lost. The heroine even abandoned the search!)
4. Preaching, teaching and patronising.
5. The author doesn't even try to find out aboutthe layout of a standard manuscript.
6. Bad German (or any other language, for that matter), wrong spelling and grammar.
7. Corrections inserted by hand (!) and the manuscript still contains mistakes. (Makes you wonder what a job application by said writer would look like).
8. Poorly edited texts (e.g. sentences that are too long and meandering, unnecessary adjectives and adverbs).
9. Fantasy worlds that follow no logic and no rules (and are therefore robbed of their believability and tension).
If you consider writing for children and young adults, you'll have to be aware of the following points: The children's and YA market is the hardest and most competitive of all, and the audience the most demanding and least forgiving. Children won't finish a book out of nicety. One boring passage and that was it.
You don't write a children's story out of boredom on a rainy Sunday afternoon. Children's and YA literature isn't a shortcut. You write for that market because you have a passion for it and are (slightly) mad. And not because your writing isn't good enough for the adult market.
Well, I'd better listen to my own advice and stop preaching. Still interested? Good! I'm looking forward to hearing from and about you and to reading your books. Good luck!