All too often when it comes to seo advice is given as general facts when in truth the advice may apply mostly to one search engine and not any of the others. The major search engines are different and do look at websites differently and place greater emphasis on different things.
Aaron Wall of SEO Book has written an excellent article comparing Google, Yahoo, MSN, and Ask across a variety of factors that help determine how each engine will look at a given website and web page. The article Defining Search Engine Relevancy is well worth reading if you want to better understand how each of the four search engines will see your site and what things you can do to improve your rank in each engine.
Aaron’s article has a lot of great links through it which provide a wealth of information and again I urge you to read it. It is a long article so be prepared for spending some time with it, but I promise you’ll be glad you did. It’s easy to make too many generalities about the best way to link or how much to put into on page seo, but it’s important to remember that search engines are different and tactics that will have no affect or even cause harm in one search engine may actually work very well with another.