Functionality, in best case, is suitable for IT people. For non-IT users this part of Mantis is:

• looking terrible
• difficult to understand
• difficult not to make mistake with commas and field names
• custom field names look even worse

I suggest:

• [major] replacing current functionality interface with something 'drad-able', like http://jqueryui.com/droppable/
• [minor] Displaying (custom field) Field Name instead of custom_Field Name