Have a lot of US-centric traffic. They have criteria on their websites, but even if you have a lot of traffic, if it isn't based in North America, you'll have a hard time. There is at least one company that converts UK traffic, but for the most part, it's all about North America. BP
Yes, you need a website that targets North America and also a great deal of traffic. I signed for fastclick and got rejected, I don't worry, adsense works fine for me.
I tried signing up for casale when my site was getting 5k daily visitors... not accepted. Tried again at 8k visitors... not accepted. Tried again at 10k visitors, they finally let me in, and boy, am I happy, their rates are amazing.
Traffic does matter: Use social media to get traffic, after getting traffic you will get good result on search result. SERP's will help to generate lead. Then apply to any CPA Network. It will accpeted for sure.
You usually must have 500,000 unique visitors per month for acceptance into these tier-1 cpm ad networks.