I read this essay on Kinetic Energy, Flywheels, and Friction at A List Apart when it first came out last year. I just reread it. Here's the basic idea:
The reason you have a Web site is to get visitors to take some action. In order to induce them to do so, you have to give them enough momentum to get them through the process--with all its attendant friction.
There's real power in this. Retail Web sites, for example, are full of friction and rely on their customer's determination to get the product to overcome that friction. Too often they have more friction than is necessary. For example, when I go on the Web to buy a Utah Jazz jersey, why don't sites guess that's what I want based on the fact that I'm from Utah? Most treat you the same regardless of situation. That's excess friction.