I keep reading about OGL material that is written in such a way that makes it harder to use than normal, and other OGL material that is written such that it's easier to reuse in other products. Specific examples I've heard are that the OGL content in Iron Heroes is particularly hard to reuse elsewhere, and that the monsters at Sean K. Reynolds' website are particularly easy to reuse.
As I understand it, one of the straightforward ways a publisher can make their OGL product hard to build on by third parties is declaring too much, or key parts, of the content to be Product Identity, whether intentionally or accidentally. How does that work exactly?
What other things can publishers do or have done that make their OGL products hard to extend by third parties, deliberately or accidentally? What can publishers do or have done to make their OGL products easy to build on by third parties? What pitfalls are there in setting up an OGL product and its license that can make it hard to reuse when your intention is to make it easy for others to extend?
The ideal answer would be a discussion of things to do and things to avoid—and why—when making an OGL product that is intended to be friendly to third-party reuse.