A method to allow a spreadsheet (MS Excel or OO Calc) to extract specific data from specific accounts, such that the spreadsheet can be used to provide different views of the available data.
For instance, I use GnuCash to keep track of my investment properties, but the only way I could figure to set up the accounts is by what each account was for (Expenses > Maintenance; Expenses > Bank Transfer Fees; etc.), with each property having a sub-account (Expenses > Maintenance > Property 1; Expenses > Maintenance > Property 2; etc.). Thus, I can't gather together from within GnuCash all the relevant data for just one specific property. With an API that allowed a spreadsheet to extract specific data from within GnuCash, I could break out the data for each property, allowing me to see in real time how each property is performing.