In the past 2 years, a very important factor in web content writing was keyword density, with the recommended 2 to 7% agreed by most seo experts. Well, that has changed for 2007, at least as far as Google goes. Their new patent clearly indicates the massive role that Latent Semantic Indexing is going to play in how they evaluate the relevancy of any given page and therefore its rank by closely considering co-occurrence rates and semantically related phrases, especially focusing the attention on the first one.

Here is an excerpt of the patent filling:
"The system is further adapted to identify phrases that are related to each other, based on a phrase's ability to predict the presence of other phrases in a document. More specifically, a prediction measure is used that relates to the co-occurrence rate of two phrases to an expected co-occurrence rate of the two phrases. Info gain, as the ratio of actual co-occurrence rate to expected co-occurrence rate, is one such prediction measure. Two phrases are then related where the prediction measure exceeds a predetermined threshold. In that case, the second phrase has significant information gain with respect to the first phrase. Semantically, related phrases will be those that are commonly used to discuss or describe a given topic or concept, such as "President of the United States" and "White House." For a given phrase, the related phrases can be ordered according to their relevance or significance based on their respective prediction measures."

So basically, anything that help determine the topics, contexts and themes of a given page, like industry terms, synonyms, buzz words, acronyms, etc, will be more than ever, very useful and impact the way your page gets ranked. The relevancy of theme based words will be playing a key role this year and beyond. New factors such as LSI and theme based relevancy were not even discussed two years ago.

On a final note, on-page optimization will become as critical as the off-page, which will have to be properly implemented so they complement each other in a new way that satisfies both your visitors and the search engines. Every person interested in achieving top ten rankings under the new LSI-driven environment should understand the basics of this methodoogy, and how to comply to its requirements.