What do reviewers look for in a paper? This may be a naive question but it would greatly assist me in preparing my first paper for a peer-reviewed journal.

I can identify at least 4 papers that I can extract from my recently completed PhD dissertation in the social sciences.

What do reviewers look for in order to assess that a paper is suitable for publication in their peer reviewed journal?
Generally speaking, they are looking for something new which contributes to the current literature of the field.
If your paper fits in this context (i.e. you believe your paper will enhance/improve current methods or even solve particular problem, then you got publishable work regardless of the reviewing output.  

For a particular journal, other factors are added to the significance of the work (for example journal scope, writing level, paper format and methodology).