One of the biggest problems with written proposals is the lack of common perception of tools or features. When a proposal is written, it is usually at such a high-level that concepts and features are understood differently. Whenever I write a proposal I can pretty much see what the feature or system might look like. From the client's point of view, they probably cannot. For this reason it is just as important to list what is not included along with the features or services that will be included. You'll be surprised by the dialogue it starts and clarity it provides.
For instance, most social applications have internal messaging. We usually recommend against it, because the effort to create a messaging system is pretty intense and most social tools should encourage public interaction. If we wrote a simple proposal that mentioned member profiles most people would assume that messaging is included. By listing this as a feature not included, we can be up front and avoid confusion later on.