John Maeda has theorized the Laws of Simplicity in an eponymous book, a must read. His 10th law is describing the concept of Simplicity as "subtracting the obvious, adding the meaningful".
An interesting application of the Law may be found in the Fast Company magazine recommended by my friend Chris. This interesting article by Cliff Kuang about the importance of taking account human behaviour in the design of commercial web sites. He relates how a major e-commerce company made minor change to his website that drove to major impact (in fact, $300 M rise in turnover).
What was wrong with the site ?
Nothing apparently but, after having made your purchases you were asked validate by a click on the checkout button then to login or register. What did the customers say ? We're not here to enter a relationship and later receive tons of newsletter but just buy some stuff. Many customers could not remember their password from previous registrations (how many password do you need to survive in the web 2.0 ?). Interesting facts : 45% of users had multiple registrations !
So what was done ?
Just change the Checkout button by a Continue one. Adding the message "You don't need to create an account to purchase products. Simply click on Continue to proceed to checkout. To make your future purchases even faster you can create an account during checkout".
Make it simple.
Don't bother your clients.