Epics are quite nice to structurize a project but Jira still lacks in transparency and usability concerning the link process and reporting capabilities. Here are my suggestions:

• Links between issues and epics should be visible in the "issue links" section
• In the "plan" view of the Rapid Board issues linked to an epic are only visible as long as they are in the backlog or in the current active sprint
• Adding an issue to an epic should be possible using the "link" command additionally to the existing "drag and drop" functionality in tha rapid board