One would have to either be delusional or have enormous pockets (Microsoft) to call their product an 'iPod Killer'. Nothing on that list will even touch the iPod, most of them look like they'll be in the discount bin of BestBuy by next January. The Zune might once Microsoft gets around to Zune v2 or v3. But I don't think anyone could take their exisiting products and think to seriously compete with the iPod, even with the same amount of marketing dollars.
iPod's most major appeal is its incredibly simple interface. The scrolling 'clickwheel' interface is so easy to use and makes finding any song a brief endeavor. The problem every other manufacturer has is emulating that interface without breaching Apple's patents. No one even dares copy Apple's 'clickwheel' in fear of a major lawsuit staring back at them. Thats why the closest you'll ever see is the 'vertical scrolling pad' like the Zen Vision M, Zen Sleek and Zen Micro have. Which is nice, but no where near as quick and simple as the 'clickwheel'. So while every other manufacturer tries to shove every possible feature into their device, making it more and more complex, Apple sits still with their simple and easy device and sells them by the boatload.
iPod's second major asset is its style. Jonathan Ive & Co. are pretty much untouchable at what they do. I don't think anyone has given boring computers and consumer electronics so much style. Not even behemoths like Dell and Microsoft been able to create devices with such aesthetic appeal.
The third asset Apple has is the huge 3rd party accessory market the iPod has. While this pretty much correlates directly with market share, but it's still incredibly important when someone is making a decision about buying a new or replacment DAP. A Zen Vision M that has just a few adapters and cases or an iPod which has a plethora of accessories for it?
Now, after all of that I think marketing comes in fourth as far as importance to the iPod's success. While marketing is very important, I don't think Apple would have the huge lead it has if they were spending the same amount of marketing dollars on the same product as Creative or Sony. They might still have a lead but it would more likely be much more marginal.
Thats just how I see it anyway.