As someone who works in accountancy (as a bookkeeper/accountant) I have to be brutally honest and say that I've been very disappointed in the UI of all the accounting software packages I've used. None of them give me the immediacy or simplicity of a well organised filing cabinet, they really don't.
I'm not sure what a good solution would look like, but I can't help thinking that making the double-entry system more visible and apparent to the user would be a good start.