Most email marketing "experts" focus on Open Rate because, frankly, it's the easiest metric to raise. Very focus on Conversion Rate because that's very difficult to raise. However, it's the Conversion Rate metric that's most important, because that's how you make money.
How, then, do you ensure that you get the highest Conversion Rate? Before getting to specifics, I must quash the widely-held belief that the higher the Open Rate, the higher the Response Rate and, therefore, the higher the Conversion Rate.