What can't you do in Excel?
gcanyon: As others have written, some of the things I've longed for in Excel are found in Numbers -- they're also available in Ragtime: the ability to place multiple spreadsheets independently in a single document and work on full ranges within them. Ragtime adds rotation of spreadsheet objects.I prefer lightweight databases to spreadsheets, so my suggestions would be similar to what FileMaker says: http://www.filemaker.com/articles/database/new_database.htmlIn short: make it easy to present a set of information through different views, make it easy to share between users, make it easy to create and maintain complex data structures and validation. Presenting information from the spreadsheet through the web, with or without edit capability, would be good.