There's a Niels Bohr quote that I love: "An expert is a man who has made all the mistakes which can be made, in a narrow field." After ten years working as a development manager at web agencies, I feel like I've made more than my fair share. My talk would take a narrative approach to mistakes that I've either made or seen other agencies make in hopes that I might save my generous attendees the trouble of making them. In particular:

Define growth, and talk about when growth can actually destroy value
Debunk the American business myth, "Grow or Die"
Offer strategies for retention, and talk about why it matters so much
Discuss sales strategies for getting good clients, doing what you love to do, and making your work repeatable
Regale the crowd with epic tales of agency success and failure