Since we offer E-commerce Search Engine Optimization services we tend to get the question, "What's the most important thing we should focus our efforts on for SEO for our e-commerce site?". The answer to this isn't necessarily the same for every client since each client has a unique business situation they are in. Some clients have sites that are coded so poorly that no amount of time spent on content is going to help them, while some clients have everything in place but need help with optimizing their content for keyphrases/keywords.
However, with that being said, the one thing most of our clients seem to overlook as extremely important is the value of the Page Title. Search engines put such a huge emphasis on the title of a page, why aren't you? If you run an e-commerce business and your product pages don't have fantastic page titles I hope you run to your CMS and make some changes now!
Do some keyword research for your products and then try out different page titles to see which ones give you the best results.