Expanding on rami's answer a little, its not just clean and valid [X]HTML that counts, its also how the document itself is structured.
Here's a fine example:
<p>
<b>My main idea, which search engines will treat like any other content</b>
Wow, all of my poor text is completely out of context :(
</p>
Verses
<h1>My main idea, which search engines will treat as a main idea!</h1>
<p>Now search engines understand my supporting text even better!</p>
<h2>Use tags as they were intended</h2>
<p>And all of your text is indexed much better</p>
I see that mistake made more than most others. Services like w3c should be able to extract an outline of your document, based solely on your tags and it should directly line up with what you are trying to present.
Wordpress does well not just because of SE friendly URL's, meta tags, etc .. it does well because it organizes the content of each page correctly with not just valid, but semantically correct XHTML.