In addition to the many good points above, I'd emphasize getting very specific on the people that will be working on your project. An agency will only be as good as the talent on your account, and since turnover at agencies can be very high, looking at past work may not always be predictive of what you're going to get.

So ask to meet with the people assigned to your project, and ask to see their specific portfolio of work - that will give you the best sense for what you are going to get. I'm happy to speak more about this and review your proposals if you'd like.