The primary obstacles that good documentation must overcome ultimately derive from the needs of the reader. The reader's purpose is to achieves success with your product (e.g. a machine, or a tool, which most software is).
1. The reader has limited time to achieve initial successes with your product before he gives up and starts looking elsewhere, either to other documentation, or to another product.
2. For any given question the reader is asking "on the path" to that success, the reader has limited time to get it answered before he gives up, and again starts looking elsewhere.
