Besides my own thoughts, nobody has influenced more my design decisions on Sampa than Don Norman. Don't know him? He is an expert in user experience and usability. I read his book Emotional Design when I started designing the base of the Sampa UI.
I never really got it why people think that Apple's Mac or iPod are better because they are simple. I absolutely disagree with that. One mouse button cannot be easier than two. A wheel is not easier than buttons (don't ask a teenager, but somebody that never ever used an iPod).
I do get 37signals way of life. It has served them well, their customers well and it has found a niche, but they will never be a Microsoft, a Google or an Amazon in terms of revenue, users or usage unless they change their vision, which I don't think is about to happen.
I'm a big fan of giving the user the control of everything. The secret is how you surface the important features, hide the advanced ones and still make the system not overwhelming for new users.