Knowing who is reading the document is very important. The requirements documents somehow needs to be from any technical terminologies, because usually if you are making a tailor made software, the client is usually someone who is out of our field. So be sure to determine the audience. So always use as much simple words as possible. It is not a design document, so don't include diagrams that any of the stock holders will not be able to understand.
Logical workflow of the business process
You should write a logical work flow of the business process, and all the system pages or frames should be integrated with each other. Think of every single action the user could do, and the way to do so, and how this action will reflect in the results, and on the other parts of the system.
The User Interface
Please note that the most important thing (sometimes the only thing) the client cares of is the user interface. You may have done all the functionality to work perfectly, but when it comes to the UI we should take care of it. In order to save time, you should settle all the UI issues with the client in the requirements document.