I got the iPod the day it came out in the UK (2001-11-22, London MacExpo, Islington), plugged it into my Mac, and in short order had all my iTunes music on it, including playlists (remember: iTunes was there before the iPod; Apple even had a "Rip, Mix, Burn" commercial [1]).
Sure, at that time, few people had a Mac, but certain demographics had it. And I distinctly recall that people in the office were curious about the iPod, and were impressed with the intuitive UI (with the physical scroll wheel, which was really quite nice - it had inertia and would spin a while when you spun fast).
FireWire, integration, and storage size ("all your music in your pocket") definitely played a role. Basically, if you had an iTunes library, the iPod was a total no-brainer.
[1] https://www.youtube.com/watch?v=4ECN4ZE9-Mo