Apple got to #1 by being better
When the original iPod was introduced, it was much better than the competition. I know, I've had an original Creative Jukebox - nice mechanical quality, but large and with a horrible user interface. The iPod was a success before ITMS, too, so the "seamless buying experience" came later.
Apple also has the slickest looking MP3 players, and looks do matter. Plus, the competition is stupid now - for example, Samsung started out by pricing their Nano clone the same as the iPod Nano. Geez, which would I buy?
Actually, neither, when I replaced the Jukebox, I went cheap (I'm a married guy now) and picked up a refurbished m:robe MR-100 for much less than a Nano or Mini; it works well for me, especially since its playlist format has been hacked. As far as the others go, Creative does seem better than most; I do like their replaceable batteries. But the Micro was too pricey for me and funky looking.
If m:robe dies now, I'd look at the Sansa e2xx series - if and only if Rockbox is ported to it.
I think one of the best MP3 players is the Empeg/Rio Car, which runs on Linux, but that's for use in a car or at home. I'm hanging on to mine.