One of the most easy SEO issues I'd pick up on a pitch in first half of the decade was the canonication of domain names. Now it's a widely known issue. Hurmph!
The with-www and without-www issue doesn't just effect search rankings directly. It effects social media (and therefore search rankings indirectly via quality signals). Look at how NaughtyCode is splitting it's 'vote' in delicious by not having sorted out it's sub-domains.