SEF URLs, well-formed, clean, non-bloated HTML, redirecting individual post links to the canonical thread URL, use of the rel=canonical tag, use of meta description FWIW, use of the <h1> tag.
On the down side, the <h3> tag seems to be misused for conversations/alerts/new users popups. I say misued, since for SEO purposes, teh h* tags should be for content, not for boilerplate/chrome stuff.
Also, for SEO and usability purposes (screenreaders, etc), the site chrome (header, navbar, etc) should be at the end of the source HTML, not at the beginning. For normal display, they can be moved to the top with CSS. Look at the source of a Wikipedia page to see what I'm saying.