The biggest thing I'd like to see more often are usable example applications. Not just snippets of example code, but working applications which show off some aspect of what your library or tool is intended to do in a complete form. And keep in mind that people looking to see if they want to use your stuff see buggy applications, that reflects on
everything.
There's a joke to the effect of "Every working Unix program derives from 'Hello, world!' in K&R." There's a similar joke about Windows programs and the first example program in Petzold's book. Write your library or tool's version of a 'Hello, world!', make it tasteful, and keep it working across bug fixes and version changes. That's the testbed; that's what people new to your work will modify stepwise into whatever they really want to build.
The second thing I wish I saw more often are "Theory Of Operation" documents, which document how the developers expect their stuff to be used.
Who is the intended audience? What other stuff do the developers expect the intended audience has seen? If it's a library, what do the developers expect the core of a client application to look like? If it's a tool, what do the developers expect its most common mode of interaction to be? It shouldn't be a listing of what each function does, but something that gives an idea of what the developers think the core functionality is would be very helpful.
Get into the head of your users. What would you want to know going in, if you were a user?