Daring Fireball: iPhone-Likeness

Figure out the absolute least you need to do to implement the idea, do just that, and then polish the hell out of the experience.

-- John Gruber on iPhone Application Design. In a world where services are moving online, and people aren't so much buying your software as choosing to use your service, I'd argue this applies to not just the iPhone but almost any "non-sovereign application" -- pretty much anything short of a word processor, web browser, or spreadsheet program.