If you have a site which equally covers 4 parallel areas and you put the same amount of content up for each area odds are that one area will drastically outperform all the others.
I have a 1,000 page AdSense site which gets about 100 ad clicks a day on the most profitable page of the site, while the site as a whole averages about 1 click a page.
The reason one section will drastically outperform the others is not just due to how well that section is integrated into the web, but also due to what types of sites you are competing against. For example, many large corporations still do not get SEO, and in certain high margin verticals (especially with certain high paying keyword modifiers) the top ranked results are dominated by cheesy spam pages. It is going to be easier to outrank the cheesy spam than to carve out marketshare in results dominated by legitimate authoritative sites.
After you get the base of your site up listen to the feedback the search engines provide. They will tell you what they think your site is about and what sections they think have enough authority to merit a top ranked position.
And while this advice can sound like it is geared exclusively toward spammy AdSense sites, the same type of market feedback, if tracked, can be applied to improving lead quality for smaller ecommerce sites. Every market has gaps in it. If you create a base of real content and listen to your site you will find enough easy opportunity to create a revenue stream which builds up a profit stream that allows you to invest in building up the authority of your domain for more competitive queries.
Aaron Wall is the author of SEO Book, an ebook offering the latest
search engine optimization tips and strategies. From SEOBook.com Aaron
gives away free advice and search engine optimization tools. He is a
regular conference speaker, partner in Clientside SEM, and runs the
Threadwatch community.