Good list. For me (and I have experience on both sides) it boils down to this ... If you are going to pay a lot of $ to engage an agency, then [1] Commit the proper resources to managing the agency [2] Commit to establishing an internal process for how to interface with the agency [3] Understand (and communicate internally) the agency's role.
Great list. Now if we can just get clients to take note. 
Years ago, I co-taught a class for a couple of semesters at the Kenan-Flagler Business School with Dick Blackburn. The class was titled: Understanding and Managing the Design Process. Our point was clear: to participate (either as a client or as a creative partner) in the development of creative solutions, you HAVE to understand the process by which creative solutions are developed. If you DO understand this process, then as a client you will know when to step in, and when to let others do their job. Knowing your role, and understanding the role of others is fundamental to being an effective client. (I prefer the word "effective" over the word "good.") Understanding the process (which is a lot larger than just knowing that agencies charge for their time), and understanding that the process starts with clear goals, and understanding that no one else can set your goals for you, are all crucial to achieving measurable success.
What I found in teaching the class is that, by and large, everyone really does want to be as effective as possible, and to get the most bang for the buck. The fact that agencies don't share/teach the process as they partner with clients only seems to me to be short-sighted.
dc
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