For me, it's the seamless integration. I love the fact that after I play stuff on my iPod, it updates the play count and last played time, next time I sync with iTunes. I build several Smart Playlists based on this info, and if the integration weren't so tight between iTunes and iPod, I wouldn't get that data. Speaking of Smart Playlists, the fact that the iPod supports them is also excellent.
Most iPod competitors are only accessible on the Mac, if at all, by mounting as a disk and copying files back and forth. Not only does this take manual effort (no auto sync), but it doesn't update the metadata I want to track. For me, the convenience of this integration is by far the biggest feature.
Sure, the interface is nice, it looks nice, fashionable, etc. I do like that it's smaller and less "clunky" physically than competitors, but even that is very much a secondary consideration to me.
The one thing I do hate about the iPod is its lack of gapless playback. A search of my posts here will certainly tell you that...
That issue is also very important to me, so I have actively searched for a replacement that does gapless. So far I've found nothing that even comes close to the seamless integration you get with the iPod, and most of the competitors are sketchy at best on gapless. There's just no other game in town besides the iPod, especially for the Mac.
Please please please, let generation 5 introduce gapless as a "new feature" (even though I consider it a bug fix)!
