The overriding factor, I think, is that the overall scope of the Mac platform (and Windows, too, for that matter) has grown so large that it supports a wide var

The overriding factor, I think, is that the overall scope of the Mac platform (and Windows, too, for that matter) has grown so large that it supports a wide variety of UI design philosophies and styles. Apple has also wandered around a lot over time, especially cosmetically. Remember 10.0-10.1? The horizontal pinstripes and garish drop shadows? http://www.guidebookgallery.org/screenshots/macosx101 http://www.guidebookgallery.org/screenshots/macosx101 is an example of 10.1. Control Panel, Safari, iTunes, etc all had disparate styles, especially over a version or two. System 6.0 -> OS 9 got a lot busier with no clear benefit, as well. As far as I'm concerned, one of the biggest things about Apple's human interfaces, especially contrasted with Windows is something that can't be taken in isolation in a single screenshot. Namely, Apple will readily rework a flow for a task (such as changing an IP address, for example) that Microsoft would just put a wizard over the top of but otherwise leave in its archaic form for multiple generations. Part of maintaining an Apple-like UI is keeping the visual appearance fresh, adjusting to new widths, spaces, gradients, etc. with most every release. So far the iPhone has centered around minimalism for one good reason -- most every other smartphone is horribly fiddly, with a zillion options (Nokia S60) or an exceptionally busy (WM) or crude (BB) UI. Minimal is Apple's schtick, and there was an underserved market for it right there.