I don't think the goal is to prescribe exactly the same look and feel for all apps. A useful analogy here is to look at what has happened in the Mac world (I w

I don't think the goal is to prescribe exactly the same look and feel for all apps. A useful analogy here is to look at what has happened in the Mac world (I won't use the iPhone as an example as the App Store reviewers impose HIG compliance everywhere). Nearly all Mac applications have the same feel to them - even third party apps. This is what makes the Mac experience so nice, apps all work the way you expect them too. But, the final interface can often look highly unique. Think of applications such as Delicious Library. Interface guidelines are for those parts of your app that aren't the core. They are for providing a layout that is reasonable and familiar to users when your application doesn't need to do better. For example, think of Delicious Library. Obviously, the browser screen of DL has nothing at all in common with what the HIG suggests. This is because it is the core experience of the app, and Wil Shipley felt that it was important enough to bring his own feel to the app. But open up a preference panel, and it looks exactly like a preference panel on any other Mac app. There was no need for Wil to spend energy trying to figure out how to layout his preference panel, he just followed the HIG. And now users of the application get that comforting, familiar feeling when they open the preference panel.