The ability to traverse through a formula - i.e. move between cells and tabs when selecting individual elements of the formula.
There's a company called BPM which has a Traverse feature. When you select the feature and click on the individual elements of a formula, the feature takes the user to the location of the element, whether it's in the same sheet or a different sheet, or workbook, i.e. t will open up the tab and go to the cell refernce.
This is extremely helpful for complex formulas as it allows the user to navigate directly to each element of the formula string without having to find source references across multiple worksheets and workbooks.