I would like to be able to search for issues that have

• no value set in a specific field
• the value in a number field larger than a specific number
• the value in a number field smaller than a specific number
• the value in a number field NOT equal to a specific value (could be a combination of the two previous)
• no due date set
• a due date earlier than a specific date OR no due date set

These capabilities do not seem to be there through the issue navigator.

I would recommend that you take a look at IBM/Rational ClearQuest, which provides a very flexible interface for defining searches/reports, where it almost exposes the SQL query directly. Maybe you should keep the current navigator as an easy tool for beginners, but augment it with a more flexible tool for administrators and advanced users?