The work item descriptions are, as you have discovered, not reportable items. This is because they can be quite large. As the work item is edited it can become pages. Consider using the Title in your report instead and possibly having a "drill through" to the longer description.

As an example of a report with the full work item linked see the "Bugs Found Without Corresponding Tests" report. The work item data can be accessed via: http://<Your Report Server>:8080/WorkItemTracking/Workitem.aspx?artifactMoniker=<Work Item ID>

The work item descriptions are stored in the [TfsWorkItemTracking].[dbo].[WorkItemLongTexts] table. Please keep in mind that, when and if you upgrade, you may need to slightly modify any reports you create that are backed by the TfsWorkItemTracking database.