We've all done marketing requirement documents. They get pretty involved, and when done, a great product emerges. At least if we've done our job. Then we need to communicate it crisply to others. I don't know about you, but I've seen way too many PowerPoint presentations. Which tends to be a crutch because if you have more to say, add another slide.
A very powerful technique is creating a dummy product slick. This has two advantages. First off, since you are restricted to a single side of a sheet of paper, it really helps you focus on the value propositions. The ones which really count. Secondly, you can show it to different people and get their reactions. Kind of like web analytics, but in the first person. If you are on the mark, they will get your concepts and react. If they don't, well it's back to the drawing board.
Once you have the product slick mastered, try a small post card mailing. The kind where you only get a title and one sentence to grab someones attention and present your product.
When you have this, you are ready to get engineering on board. Because they too will get it, and as they go through the detailed product requirements, everyone will be building toward a common goal. And that beats trying to mico manage a product.