Why do we need to go through this exercise? Your primary audience should be one of the biggest factors in every decision you make, so you have to have a clear picture of who they are. Take the fundamental design principles behind the front pages of two sites I use often; www.google.com and www.webmasterworld.com.
Google's primary audience includes all web users. Yes, that's how broad a scope a primary audience can take. Given that they are designing for everyone, they can't make many of assumptions with regards to background knowledge, experience with the web, etc... Given that they're a search engine, they also can't make many assumptions about what their visitors are looking for. The end result is a prominent text box to type in your search phrase and a button to execute the search. Despite having a large number of services and features, they target their primary audience properly by keeping their front page lean and simple.
The Webmasterworld site, on the other hand, targets a more specific group of people as their primary audience when compared with Google. Webmasterworld largely targets professional webmaster as their primary audience. The main page reflects the needs of this audience and shows a sharp contrast in style when compared with Google's main page. The web professionals who visit this site are often interested in current topics. The most important topics are selected by the administrators of the site and are displayed prominently on the front page. The other major feature of the front page is the complete listing of the many forums they have. This caters to their primary audience because most web professionals have interests in certain areas and will be interested in a subset of the available forums. However the areas of expertise and interest vary greatly from webmaster to webmaster, so all forums need to be listed in order to accommodate all of their primary audience.