It wraps your music in a DRM whether or not you want it to.
This is sort of a far fetched scenario, but imagine you've got a band called, oh, Admiral Viscen and the Love Squadrom (Heretofore: AVLS). You just put out a record yourself, on your own dime and released it on your own label. The music belongs to you and The Squadron. So, you're on the train and you spot someone else with a Zune and you think, "Oh, great! I'm gonna beam them our new record!".
Well, tough tits, AV. Whether or not you like it, your music is wrapped in the 3 days/3 plays scheme. Even though you made it.
Other complaints are that the software is generally sloppy, no OSX support and the WiFi is generally wasted (No WiFi store support? WHAT?!). I'm sure it plays MP3s just fine and if you've never used an iPod, well, you'll be okay. But the iPod is just a device that does one thing so incredibly well that everything else that follows seems like a fuck up. K.I.S.S., you know?