See the title of this post.
A brief is digested, in all senses of the word -- a condensation of thought that indicates a clear strategic direction. Killer creative thinking comes from focus. You need a bull's eye to aim at, not a dartboard.
In my experience, there is an inverse correlation between the length of a creative brief and the quality of thought that goes into it. I have fond memories, as I'm sure we all do, of reviewing a brief in abject fear, realizing that the client is requesting work that communicates three or four different ideas within one piece. Many of them can be contradictory.
This is when you need to work with your client to narrow their strategy to the right key message -- one that is easily communicated and makes sound business sense.
The creative process can be easily co-opted for the express purpose of focusing the client's marketing or branding strategy in the work itself. This is a waste of time and money for the client and the designer, and no way to run a profitable business.
Ideally, a brief should never run longer than 2 pages. Everything else is just incidental detail. Do all those facts and figures and charts need to be in the brief? Can you communicate them in another way or provide them as supporting material? When I see a thicket of details in the brief, they are often secondary to the goal of the project. Everyone needs to share the goal before the details can fit.
The real litmus test to a compelling brief is when you polish up your designs, board them, meet your clients in the conference room, and introduce your work by sharing one to two succinct sentences that summarize the entire strategic direction of the project. At this point, their heads should be nodding expectantly, as they wait for you to reveal how you've clothed their business needs in compelling artistry.