The two mistakes that come to mind are people who introduce a flood of characters in the first few pages. Where the reader has to stop and get out a flow chart and has to figure out who is who. And you just can't do that - introduce the first four generations of a character's family in the first chapter. You can introduce four or five characters at the most in the first chapter. Another mistake is to use big words that are not normally used in conversation to try to impress folks with your vocabulary.