- none of your current "plays for sure" purchased music will play on it, because they're using a different DRM scheme;
- if you want to buy music that will play on it, from the Zune Marketplace, you can't do that with real money, you have to buy Microsoft Live points and use those;
- the points are 79 for a dollar, so you can't easily calculate the price of anything you buy;
- even though it has WiFi built in, you can't purchase and download songs using it;
- you can share songs with your friends, but once someone "squirts" you one, you only get three days to play it and at most three plays, then it's unusable; and
- there is no simplified way to buy such songs if you decide you like them.
Apparently, something on the order of 40% of artists fall into this category, but you have no way of knowing which until you've already bought the songs and try to share them.
Yep, this is definitely an "iPod killer" :-) !!