I'm a little disappointed how the top post in here presents design almost as an opposite to usability. That's not how it should work and certainly not how Apple got so successful. I think you might mean "aesthetics", that would be an important difference.
Good design is never just decoration, it's not just an aesthetic hull with the "real" functionality hidden behind it. Good design is from the ground up, how the entire experience is structured, including the stuff under the hood. Apple's main strength is in putting a ton of research into how users actually use their devices (textbook "usability") and making sure things are always ranked by importance. There's stuff iOS doesn't do that most Android devices do and it's not missing because Apple can't pull it off, it's because they decided not to waste space/learning-time for the user on something that might, for most people, not be important. Meanwhile, niche functionality is supported through a myriad of apps. That's not minimalism for the sake of aesthetics, that's a really smart usability decision that has payed off over and over for them. For a counter-example, I'm sure Apple was very tempted to make the Apple Watch touch-only but instead included a somewhat clumsy looking scroll wheel because that makes more sense for such a tiny display.
The few places where aesthetics clash with functionality (a lack of physical buttons that might seem reasonable, some questionable font choices,...) tend to get replaced by a more all-around solution (forget about buttons and go straight to touch) or a willingness to identify and fix problems (a new font for Apple Watch and iOS9).
I really don't think we'll get very far by calling out usability, of all things, as an area where Apple lacks. Apple has other problems, they're not perfect. But if they fail, it'll probably not be because of usability issues. It might rather be that people start asking what reason there could possibly be to upgrade to the next iPhone.