What's the one crucial Search Engine Optimization mistake made by many of the most prominent Joomla sites?

Mistake: Mis-using Global Configuration metadata. In the last few weeks I've seen many websites harm their SEO chances by having the same description and same keywords for every page.
 

Its tempting to fill in those boxes, but think about it this way....Is there really a description and keyword set that can apply to every page on your site? Does you frontpage really contain the same information as your site map?  Does your "Contact Us" page really merit the same description as your forum pages?

Every template club I visited made this mistake. One of them (no names) has 516 keywords, each one of them appearing on every page. 16 might be a better number. Needless to say, I couldn't find them ranking for even one important keyphrase.

Oh - and Joomla.org makes this error throughout its Main, News and Help pages.

You'll be amazed how many times you see the following source code:

<meta name="description" content="Joomla! - Content Management System and Web Application Framework" />
<meta name="keywords" content="Joomla, joomla, Joomla!, joomla!" />

Solution: Leave your Global Configuration Metadata empty. Apply unique metadata to each page.