I do SEO work for a large (DJIA component) tech company, so I'm pretty far removed from startup-land, but I'll still share the number one piece of advice I give out internally:

Write high-quality content about topics that people are searching for.

Simple? Yes. But it still seems so hard to get people to do it. Everyone would rather talk about page load time and HTML markup and structure and meta tags and exactly how many characters their headline or URL should be. Yes, you should make common-sense decisions about those things. But no amount of technical optimization is going to make people start Googling something that they couldn't care less about to begin with. Find out where demand exists for answering questions, and give the user a good experience as you answer those questions, and the search traffic will follow.