Recently Google announced they were changing their algorithm for displaying search results. They are going to start penalizing sites that have content they view as "spammy".
Google's Principal Engineer and anti-spam czar Matt Cutts writes on the Google blog, "...we recently launched a redesigned document-level classifier that makes it harder for spammy on-page content to rank highly."
This is a big deal to search engine experts who are constantly trying to figure out how Google ranks site content and adjust accordingly. There are other search engines, but Google is dominant and usually sets the standard for the industry.
There are several search engine optimization (SEO) sites out there giving advice or offering their services. In general it's a good idea to follow their lead and to build your site using best practices.
The problem is creating your site for the search engines alone. The purpose of your site is to be read by your customers, not the search engines. Search engines can play a vital role in people finding your site, but if the user does not have a good experience then it is a wasted effort. The discipline of designing for users is called User Experience Design or UX.
In fact what Google and the other engines are trying to do with their algorithms is figure out if they are sending their users to sites that provide a good experience.
Good site design is often a balancing act between SEO and UX. When in doubt you should err on the side of good UX. Providing a great experience for your customers will never be obsolete, while following a strict SEO formula could change tomorrow.