MiniDiscs???
I hope you're not serious. Sony has been pushing these things for a few years (2 or more) now and they have barely gotten off the ground. They are (more like were) a good technology, but they had pricing problems and other more conceptual barriers that stopped consumers from purchasing them.
The iPod succeeds where the MDs failed for a few reasons. First mp3s are a lot farther along right now. It is very simple to just pop a CD into your computer and mp3-ify your music. Also, you can go out and buy (or acquire) mp3s electronically and put them on your iPod. The mp3 fomat beats any Sony format because of its small size and universal acceptance: everybody can play them.
The iPod can also hold any file from your computer. Its like a Jaz drive without the disks and is much faster. I have a ton of music on my iPod, some projects for work (source code), and some of my writing on my iPod. I can bring it between work and home and listen to music while I transfer files between.
In contrast, the MDs I've seen have been small and, as far as I know, only store music. And did I mention that very few people actually use MiniDiscs and that MD's only play on specific players. iPod can connect to any Firewire port.
And they are both somewhat pricey. Draw.
I think I see a clear advantage here. MDs were marketed for a specific purpose: the next generation CD or tape. They weren't widely accepted and have become a very niche market. The iPod is just scraping the surface of what its capable of.
Matthew