The word I've always used for this is "frictionless." Decreasing the intellectual overhead to a decision or action. Every time we make a customer or prospect figure out how to do something, whether it's make a purchase, indicate a preference, or engage further with us, we introduce a potential barrier to that action, which not only decreases the number of people performing that action, but it also leads to user frustration and customer dissatisfaction. While purchasing online, how many times have you said "Why is it so hard to give these people my money?!" How many times did you throw up your hands and say "forget it?" How often has that happened at Amazon? Think frictionless. Last, don't assume that "frictionless" for the user means "simple" for us. It often takes remarkably complex data and algorithms to make things very simple for the user....

A quote I often use with my clients is, "simple doesn't mean easy." Arriving at a simple solution often takes more time and more thought than a complex solution. And few organizations have the patience to achieve simplicity. It's an ongoing quest.

Simplicity: toothbrush, umbrella, bicycle.

Simple to start. And useful. Simple is often what your audience should see...to start. Like Glenn and Lisa have said, there's more to simple than simple. The simplicity of Google is based on massive, brilliant, ever-changing complexity...that turns out to be quite useful. And it's the start (if you want to take the journey) to an ever growing set of useful services. Simple is also often the best way to communicate...to start. Attention and interest are scarce assets, and simplicity can make the most of them. They can grow, however, and providing the option of continuing the conversation and/or getting more information can truly engage people and be quite useful. Simple and effective communication can be difficult. To paraphrase many (Pascal, Thoreau, Twain, etc.), if I'd had more time, I would have made this comment shorter.

I echo Matthew and Lisa on this. It is important, but sometimes it takes time and a lot of energy. It's insanely complex to be stupidly simple. twitter, Google, Mac, iphone, Amazon, Roomba, Craigslist, Simple Human.. all proven points. Got any others?