The site age is one of the single most important criteria of the Google algorithm, especially for new sites in competitive niches. This effect is created by algorithm components rather than an algorithm component itself. This factor become increasingly more important in the future.

There's little doubt that the search engines see what users are doing, especially users that have opted-in to certain search engine products. I believe Google is still at the learning stage here. Gathering data and try to understand it. This factor is going to become important in the future.

There are multiple examples of top rankings for competitive keywords with only internal links pointed to it. Try to use anchor text links with keywords in the site menus and within the content when applicable.

More a important factor for country specific than for edu, gov, com, net, and org sites.

Good content have noting to do with HTML Validation from pages. This is and shall probably never be a ranking factor, like it should be. But the chance is that a part of the text disappears for the spiders when a tag is not closed properly.