There's a key ingredient to every top-notch advertising agency: it's all about process. As boring as it sounds, it's not enough to just be talented and creative; you've got to have a method to your madness.
Sure, creativity is key, and no one's going to be interested in an agency with a meticulous work-flow but uninspired ideas. But consistency is very important if you want satisfied clients and, especially, repeat clients.
When you meet with people for the first time and set out to discuss their needs, you have to give them a solid idea of just how it is you'll go about meeting those needs. Well, you don't just have to give them an idea; you have to be able to carry it out. Then client wants to know that you're not just going to sit around at your desk until a idea occurs to you. They want to see that you have a process, a system by which you can guarantee consistent, first-class results.
It's like in science class, when you learned about the "scientific method" and how to keep track of all the steps in a given experiment. It's crucial to have a clear, repeatable technique that you know will lead you to the same results each time. In this case: successful advertising.
Of course it's a lot trickier to come up with a consistent method for great advertising than it is to remember how to mix chemicals in a certain order or use a bunsen burner. But that's what makes one agency better than the next: the experience and honed skills to know what works for their clients.