I think that one of the reasons the Zune... or even the Zune HD is interesting, is simply because it is different. If Apple allowed interface customizations, that would knock most of the differentiation out of the equation, but risk adding complexity and unpredictability to their customer support and ease-of-use.
Based off of this video, I couldn't imagine why ANYONE would pick this device over an iPod Touch... but I know why they'd want to consider it. The web browser would HAVE to be very good, the WiFi hotspot switching as transparent (unlike the PSP), the multimedia handling (PDF, Excel, Javascript, Mp3, etc) as complete, and features like Google Maps and YouTube would need to be as straight forward to use.
The notion that the OS would not be "smart phone grade" seems immediately obnoxious. The notion that on a competitor's device I can play tens of thousands of games and applications ("Skype", "SlingPlayer", "LogMeIn", "Joost", "Voice Notes", "Wolfenstein"), purchase music/software wirelessly, and support so many iPhone features... like "email", "notes", "calendar", "contacts"... and NOT on the Zune HD is bewildering. --I mean, if the Zune is priced even remotely similar, and its missing all of this capability, its kind of silly, unless the market is truly saying that these features do not matter.
I'm not sure Apple's billion-download marketplace is saying this is an irrelevant thing. If Microsoft is holding back on another layer to its Zune HD strategy, it would be great to hear. However, if one can access "Hulu", "YouTube", and "MLB" on iPod, and not on Zune... it would take some truly stunning XBox 360 integration and 3rd party support to crank up the heat.
~ CB