Too short (<10kb), no paragraph (one big blob of text), no punctuation, all captial letters, just name a few. To be honest, I don't set the criteria very high here. I know some readers want to see more restricted approving process or even setup an editorial team to read all stories before posting. I would like to give new, amateur writers a chance to publish their works. And some of them indeed improved their writings after getting some feedback from readers. But there are alway some people don't really take it seriously. So there is always 1 or 2 rejected in almost every update.