What do reviewers look for in a paper?

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).

This is definitely not a naive question.

The best you can do is to understand the position of a hypothetical (ideal/unbiased) reviewer and immediately from that you will get answers to your question. Particularities could depend on the field (theoretical vs. experimental, etc.). Hence, look for answers to the question "how to review?". Already answers here at academia.SE provide lots of relevant points. Answers to this question are probably the best starting point.