What can't you do in Excel?
pasbesoin: diff in Excel.  Against different components, whether cell contents, formulae, cell-associated comments, etc.  If you want a headache, try supporting recognition of intracell text formatting.Rereading, I see this is a "new" spreadsheet application.  Well, hopefully then the data format and API's will be less... obtuse.Interactive, visual interface to same diff functionality.When I last looked, for Excel there was a product or two going partway in this direction; however, most seemed rather limited, e.g. export the cell contents to text and diff that.Useful for number crunchers.  Also useful for all the people who end up using something like Excel as a glorified table.  In the business world, there are endless use cases of people managing documents, requirements, results, etc. in Excel.  Providing such a "BeyondCompare" fucntionality for this content would be very useful to a lot of them,  (Caution:  You might also have to teach them how to use it, including the diff concept.  And that could be a very significant bump to try to get over.)Since Excel is so dominant, that class of people might not be your target market.  Nonetheless, I see a good diff type utility as being a real plus.I also can agree with Zain's comment regarding versioning support.And, integrated regexp support.  I wedged same in to Excel/VBA by defining a reference to Windows Scripting Host (back in 1999 or 2000).  Very useful.  A lot of problems people deal with in spreadsheets can be greatly aided by decent pattern matching and substitution.