Ability to easily add custom properties to cells and also headers and footers
At the moment Excel shares the same front-end for creating and editing custom properties through the advanced properties dialog. There is no easy and consistent way to insert this information into cells and headers and footers.
This precludes the sort of ability to set a parameter once and change multiple locations that is common practice to automate updates to Word files. I would have used this recently to send out a survey to multiple potential partners, chaning the name of their organisation and the survey reference appropriately through custom properties rather than manually editing each worksheet.