In an article called objects of desire in Technology Review 'prominent' designers talk about their favorite products (the title reminds me of the Adrian Forty text book of the same title I had to study from at IDE). One of them discusses the first (1979) Sony Walkman.

"It has simplicity of use," says Logan. "You could give it to someone who's never used one before and they can use it. You can only get that with reduction of features."

I love the walkman too, and I think it was easy to use too; especially in comparison with current digital music players. And part of that simplicity of use was the basic functionality it offered. But it might also have had something to do with the fact that the device did not have a menu based user interface, it just offers your classic 'one function one button' interaction. And the music was on a physical medium instead of floating around in cyberspace, which made transfering music to the device a lot easier than shoving mp3s from your PC to your mp3 player. But hey, on the other hand, you don't need drawers full of cassettes anymore. So I don't think it's just a matter of reducing features.

From the wikipedia page about the Walkman we learn another interesting fact. Because the first walkman was really changing people's behavior - they had not listened to their own personal music on the go before - Sony built in some features to ensure that the device would fit the social context of use.

In the UK, it was with stereo playback and two mini headphone jacks, permitting two people to listen at the same time (though it came with only one pair of MDR-1 headphones). The Walkman had a "hotline" button which activated a small built-in microphone, partially overriding the sound from the cassette, and allowing one user to talk to the other over the music. The dual jacks and "hotline" button were phased out in the follow-up Walkman II model.

Apparently people really didn't need to share their music with others, and taking off your headphones was just as easy as using a hotline button...

(picture taken from Technology Review)