Best practices for developing DotNetNuke modules dictates that we use the development.config as our web.config, which includes running our development sites under Medium Trust. This is necessary so that we don't inadvertently introduce code that requires Full Trust into our module and thereby restrict it to customers running in Medium Trust.
However, while in Medium Trust, certain data visualizers in Visual Studio don't work (namely, the DataTable and DataSet visualizers). You'll see a message like "The application you are debugging has insufficient privileges to allow the use of custom visualizers. Please see the documentation for the list of required privileges." I've been occasionally frustrated for a number of months because I couldn't figure out why Visual Studio was barking at me when all I wanted to do was use their built-in visualizer. Finally, I found this forum post and saw the root of the problem. So, now I know, if I need to debug something in a DataTable, switch to Full Trust for a bit.
Hope it helps!