1 - Making your questions far to specific, so nobody else could ever benefit from seeing the answer to that question in the future. These tend to be down voted very aggressively. So you need to try and find a more generic way of asking the question so that others might be helped by the answer as well.
2 - Just plain dumb questions. Such as 'I needz site like Facebook. Hope to make? Helpz'. If you are doing that and do not understand why it is a bad question then I probably cannot explain it to you.
3 - Making you question so that it looks like you are lazy and getting others to do the work for you. For example, questions that look like someone's homework. No one wants to spend time answering if the questioner has obviously made no effort to solve it themselves. Stackoverflow should be the last resort after several hours of trying.
The site is basically made up of professionals and that often determines if the question is considered too simple or not.