It was the user experience mostly. The ease of integration with iTunes, the one-finger operability. With great marketing. The iconic white earbuds were the key piece of "marketing", if you can call them that. But they were a true viral marketing tool. I hate them; they hurt my ears, so I use some old headphones from a Sony Walkman.
iPod just recently passed the 100 million mark in sales... A number only reached by Sony Walkman, Game Boy, PS1 and PS2 among all consumer electronics brands. DS will "copy" iPod's success by reaching that 100 million mark in under 6 years, and though its early, I think Wii will too.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
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