iOS' UI tries to emulate things we already know (through metaphors & skeumorphism). What I meant by iOS being "more natural" is exactly this - while Apple emphasizes on reminding users of things they already know, Microsoft has chosen to kill many such analogies by purpose. The Zune/Phone 7 interface tries to get the max out of the fact that it's a digital environment, where you do not need beveled buttons or stitched leather ribbons. While I think I understand why MS is doing this - increased focus on presented data, less distractions for the eye, less energy usage on AMOLED displays - I don't think I particularly like its plainness, emotionally. And I believe that many people feel the same, mabye unconsciously.
The other thing is that, on iOS, there is a context (app) for everything, with a dedicated meaning to every screen. I agree that Microsoft's approach to navigation is not bad, and it surely fares better in some aspects. But dedicated context is often lost when apps act as a "hub". I think that this kind of information presentation leads to information overburden, the kind of which prevents people from opening an app more often than once a week.
The other thing is that, on iOS, there is a context (app) for everything, with a dedicated meaning to every screen. I agree that Microsoft's approach to navigation is not bad, and it surely fares better in some aspects. But dedicated context is often lost when apps act as a "hub". I think that this kind of information presentation leads to information overburden, the kind of which prevents people from opening an app more often than once a week.