Here's an interesting metric (via Usability in the News) for testing websites:
Q: “What percentage of your interface contains stuff that your customers want to see?”
- 10%
- 25%
- 100%
If you answer a, or b then you might do well, but you'll probably get blown out of the water once someone decides to enter the market with option c.
(continued at "A really simple metric for measuring User Interfaces" at iQ Blog)
This test could easily be applied to utility-type software whether web or windows e.g. Flickr or WinZip (it would be a little harder to apply the principle to fully-fledged applications, where "infrastructure" user interface is impossible to avoid).
37 Signals calls this kind of approach "epicenter design" in its online book Getting Real - start with main thing the user will need, then the second most, then the third, and build outwards.
Tags: 
user interface, 
usability, 
design