Good points in this thread.
Another big mistake I see people make is that they design a page based on their preferences, or based on their emotional feelings, instead of by what works best.
Everyone should be working not only on testing changes to the copy, but also the layout, offer, placement of testimonials, etc.
In terms of copy VS traffic, I think it's obvious that you can have great traffic with poor copy and get low conversions. But conversely, you can have the best copy in the world and no traffic, therefore ensuring you get no sales!
I like to focus on getting my best first draft of copy online and then driving traffic so that you can start testing.