Also if you understand the project well enough to write documentation you have to have the ability to put yourself into a new user's position. Many people lack this ability and create just unusable docs.
Agreed. It's a hard skill even when you're trying consciously. One way to improve the situation is to actually give your draft documentation to a trusted user for feedback. I've been lucky enough to have an enthusiastic user in-house who has road-tested some of my documentation, and it's much better for the exercise.
