Not to be too Manichean (look it up) about it, but basically there are two types of people in the world--and by application, two kinds of agencies.
There are the complicators.
And there are the simplifiers.
There are the "no, we can'ts.
And there are the "yes, we cans."
There are the talkers.
And there are the doers.
There are the theorists.
And there are the pragmatists.
Here's my confession.
In nearly 30 years in the business, I've worked with just one marketing person and just one agency where creative people, marketing people and media people all seemed to be on the same team.