The iPod is a killer app because of herd mentality at least as much as because it's a well-designed device. To be part of the in-crowd you must own an iPod and not an iPod-alike. Couple that with a multi-million dollar advertizing budget, a more "exclusive" price (cf. Luis Vuitton bags), and Apple's public preconception as a "cool" company, and you'll end up with umpteen million iPods sold. Apple's biggest "innovation" was making the iPod synonymous with "portable music player".
If iRiver, Creative, iAudio, or anyone else had half the pre-existing clout or marketing budget that Apple has today, the iPod wouldn't be so lonely at the top. Apple's marketing folks did a great job peddling a relatively expensive, feature-poor product with a "good enough" interface. They knew their market well and siezed the opportunity. Credit where credit is due.