I guess everyone has their opinion of what makes a good post. There is no general guide book. And I'm sure members consider different factors. Such as effort, relevance, and uniqueness of the post. Give merits to posts you think are deserving of it.

A translation of a relevant post could be given merit(s) just for the effort put in, even though it does not offer any new idea.
Members also judge ranks differently; i.e what would earn a newbie/jr member a merit might not earn a full member or hero member any.
One accepted rule would be not to use it for personal gains; on alt accounts or merit sale.