Certain types of searches distribute the bulk of the leads to the top few listed sites, while other types of searches distribute traffic further down the search results.

Brand Searchers Don't Search Deeply:

Branded search queries, for example, will deliver the bulk of the leads to the associated brand, especially if that brand sells directly and/or if focused on SEO. Generic searches and highly commercial searches will also typically deliver the bulk of their traffic to the top few results. If the search is generic in nature people will likely click one of the top few listings if it is relevant or search again. If the search is highly commercial in nature the odds are pretty good that the ads will be more relevant and more appealing than the organic listings.

Even if the brand searcher does search a bit deeper, their intent is usually closely aligned with the core brand they were searching for. It is hard to switch their reptilian mindset to do something else.

Long Tail = Deep Searcher:

Longer search queries and research oriented searches will likely yield a more even traffic distribution to sites listed lower in the search results. In some fields affiliates are all fighting to rank the exact same set of data, but in those same fields if your site has original user generated or editorial content it is easy to match many long tail search queries.

Lower Ranking = More Traffic:

Why is it worth considering this? When you look at keyword tools, a keyword with 10% of the volume may deliver more traffic to a #7 ranked site than how much traffic a keyword 10x as popular would to a #4 listed site.

The Mindset of a Deep Searcher:

Certain classes of search and types of search promote deep searching, while others are very top heavy. For example, a better way to play branded terms is to focus on coupon related searches rather than the core brand. Searches for coupons promoted in checkouts will dig through the results if none of the top ones are relevant because those people are already somewhat committed to a checkout and are committed to doing more to save a little bit.

Look at what you are already ranking for and getting traffic for. It may make sense to go for more long tail variations before going after broader and more competitive terms that you may not be able to rank for and may send less traffic even when you do.