What can't you do in Excel?
GHFigs: An alternative question might be: what are you doing in Excel today for which Excel was not intended?I remember Wil Shipley (of Omni, Delicious Monster) talking about how they originally developed OmniOutliner after noticing that most people were using Excel to make simple lists rather than complex spreadsheets, so there was a market for something less expensive than Excel and more tailored to the things people were actually doing with it.  With a product as broadly installed as Excel I'm sure there's still under-served niches.