It's the first thing every new user looks for, the raison d'être of every project, almost always will completely divide the people evaluating your software & is the leading cause of liver cancer in the American pub... wait, no. Forget that last part. What I'm talking about is the Tutorial.
Frequently the first bit of documentation written, the first one to fall hopelessly out of date & the one *everyone* sees, the Tutorial bears the brunt of getting people started. Its job is to pull people in. It teaches them not only what the software is about, but *how* it should be used. It sets the stage, the standard & the lowest bar of entry. It's unenviable because it must do so many things & do them well to be a success.