Position of Content Elements
Content? Everyone can find the Content?
The importance of the positioning and delimitation of content is immediately obvious to any one who has seen a site that doesn't do this well. Ever just been frustrated that you can't find what you think should be the "heart" of the page? Than you've run into this problem.
Once again user expectations play a big role in this. Content is expected to be in the middle-center of the page, and clearly marked out. It is expected that all the ads, navigation, etc. will surround the content leaving the juicy core of the page in the middle unscathed. This can be drawn from the same studies used in the Navigation Position Section. Although some of them don't evaluate content explicitly (like the Bernard studies) it can be inferred because they look at almost every other element leaving only the content. If you look at the spaces people have identified for other elements you can quickly see there is a blank area in the middle of the window, this one blank area is where the one missing element must go.
Separation of Content
Just as important as layout here is clear demarcating of the main content. More often than not content is placed in the center of the page but not clearly marked. Sometime it blends in with the ads (nice for advertisers, bad for users), sometimes it seems like it's just more headings/titles, etc. But what ever the reason it needs to be clearly afforded the sense that it is the main content of the page. This can be done with font type, size, and color, it can be done with a change in background color, or putting a frame around it. The proper use of all these techniques is a topic that is covered in great depth in many places both with in web research, design research, and psychology research, and is much too massive to explain all the ways it relates to web design here. Needless to say while these elements play a role in almost every part of a web page, they are particularly important tools to clearly differentiate your content.