What Makes a Good UI?

OSXCPA asks: "While there are plenty of OS business apps from accounting to ERP, they seem to share a common failing with "commercial" software - the user interface is terrible! Has anyone seen an application that has a UI that made you sit up and stare in amazement at the simplicity and effectiveness of it? For the techno-elite, drooly-gui may not be a priority, but I am working on a project (OS) where I have to show real savings (in task performance time and reduced data entry error) on a specialized accounting system via better UI. Am looking for some inspiration. Any ideas? Projects? Books?"