Using story points
You can prioritize issues and follow iteration development using story points, which is used in some agile processes, like Scrum.
First you need to add the issue field "Story points" to the issue field configuration for your project.
Go to General - Settings - Issue configuration - Issue fields, and select the issue field configuration in question.
In the "Edit issue field configuration" page, add the "Story points" field to the configuration.
Now you can set story points for an issue, and see the totals in the project page.
The burn-down graphs in the sprint/iteration page, Reports page, and the burn-down portlets should now offer you the option to show the burn-down in story points. (When an issue is completed, its story points will be subtracted from the current story point total, helping the burn-down reach zero.)