I remember when the iPod first came out it was hailed as pretty revolutionary in terms of its sheer size, something that Apple is apparently still keen to try and capitalize on (mini, nano, etc.)
At the time, I owned what was probably the very first hard-drived based mp3 player, the Creative Nomad Jukebox (I think that's what it was called). The idea of being able to carry that much music (6 whole gigs!) was revolutionary and appealing, but the Jukebox left a lot to be desired. It was huge, for one -- like an early-model discman. It took 4 AA batteries, that lasted maybe 6 hours altogether. Song transfers were done over USB and were very slow. And every time you turned it on, it had to boot up the firmware, load up the song database, and so on -- which consumed lots of battery power. I swear half of those 6 hours of battery life was used up just waiting for the thing to load. Oh yeah, and if you left the unit idle for longer than a few minutes, it shut itself off again...
So, while I loved the concept, the Jukebox quickly became impractical and I gave up. Then my friend showed me his iPod, which had the built-in rechargeable battery, fast Firewire interface, and most importantly, had a sleep mode! Not having to restart the whole machine every time you wanted to play music made it a big winner, in my books.
I think momentum has just carried it on ever since...