Do you want to rent your music?
For $15/month you can load up a Zune with a wide (although not exhaustive) catalog of rental music. Your music plays back for the length of your subscription and your credit card is automatically charged each month until you cancel. Of course, for the same $180/year you can purchase quite a few iTunes albums and singles (or go completely wacky at a used CD store or swapping service like lala.com), but you can't jump onto an actual iPod subscription model.
Subscription models are surprisingly nice. All the music you want, when you want it, without much in the way of limits. If you feel like listening to Nirvana one day and Barry Manilow the next, a subscription model means you can sample without commitment. It's a perfect match to the "for now but not forever" mindset. Sure, if you find something you absolutely love, you can go out and buy it but subscriptions give you the aural equivalent of an all-you-can-eat buffet (and, sometimes, a similar need for antacid.) With a subscription, Billboard's top music can always be in your pocket.
If an iTunes subscription model was available, would you be willing to try it out? How much would you agree to spend per month? Less than Microsoft's $14.95? More? What dollar amount would you put on such a service? And if iTunes left out album-only tracks the way Zune Marketplace does, would this be an insurmountable barrier to you?
