You don't sound different
While agencies pride themselves on strategy and messaging, few are actually any good at differentiating themselves from one another. Passionate, full service, award winning, nimble, effective, results -- they are all just throw away words when every agency uses them.
Find a way to say and show something truly different. Think about what really does make you different. Stop trying to be all things to all people. Stop pretending that you are world class at everything.
And never under any circumstances use the sentence "We really want your business…" in a document or meeting. What, as opposed to the other agencies in the pitch who don't really want it? What clients want to know is if you deserve it.
You didn't read the RFP
The documents clients send to agencies are meant to be devoured, not ignored. When it comes time to pitch it becomes abundantly clear when at least some of the members of a team didn't bother to read and understand the briefing material. While a few companies simply slap together an RFP, most really take time to provide essential facts and detail about what their businesses need.
The corollary to this is that you need to answer all of the questions outlined in an RFP, and do so in a manner that makes finding and consuming the information easy and efficient. For example, I recently saw and RFP with 18 questions in it -- some specific to the brand and some more "boilerplate."
A surprising number of agencies failed to answer every question, or to provide documents and presentations that made clear the location of each answer. When you are responding to an RFP, remember that the clarity of your response sends a strong signal about the effectiveness and efficiency of your organization.