Minimalism and skeumorphism are not mutually exclusive.
Skeumorphisms are adopted on UIs, a priori, to give hints about the function and ease the interaction with new users. It's a metaphor to reuse prior knowledge of how things work in the real world.
What Apple did with the calendar app is not skeumorphism: it's just kitsch. While the app looks like a calendar, it doesn't work like a calendar (you don't flip between months, for example), so it completely misses the point.
On the other hand, you could design a calendar that works like the real thing, using minimal lines and flat colors, and you could argue that it's minimalist and skeumoprhic (because it still carries assumptions about another object, the paper calendar, even thought flipping pages is not a metaphor of digital media).
BTW, the definition of skeumorphism in the Wikipedia article is too narrow, and at some points downright wrong.
I think Don Norman's book "The Design of Everyday Things" is largely about exactly this, and it could even provide a simple heuristic for whether or not a specific skeumorphism is worth keeping or just kitsch:
Does the additional design (whether a gradient, bevel, shadow, etc) indicate an affordance? That is to say, does the extra stuff that is being layered on serve to tell the user something about what the UI element is for. For example, a slight gradient and bevel on a button informs the users that it is meant to be pushed, as it looks a bit raised up relative to its surroundings.
This, to me, is the biggest thing missing from these flat designs. There's nothing left to indicate whether something is a button, a panel, or whatever else. It's been stripped down so far that crucial UI hints have disappeared. As other commenters have pointed out, I think moderation is the key.
Actually, both the OS X and iOS calendars do support flipping between months with a swipe. Arguably, the leather effect at the top aids in this by reinforcing the physical-analog affordance, though the torn edges are a bit much.
Now the address book, that's a nearly perfect counterexample. Once again, on both iOS and OS X, it has "pages" that invite a swipe – except here, swiping does nothing. A dead affordance that only serves to frustrate and confuse the user.
The new address book is an absolute usability disaster, IMO. Though to my great joy, Mountain Lion at least restores a large part by bringing back the three-pane setup.
> Minimalism and skeumorphism are not mutually exclusive.
> Skeumorphisms are adopted on UIs, a priori, to give hints about the function and ease the interaction with new users. It's a metaphor to reuse prior knowledge of how things work in the real world.
Indeed. It gives hints and makes elements stand out. Take the Nest presentation from the examples in the original post—it uses realistically looking elements to make them more noticeable in contrast to ‘flat’ surroundings. This helps highlight the design of the Nest thing and link to the store, for example.
Skeumorphisms are adopted on UIs, a priori, to give hints about the function and ease the interaction with new users. It's a metaphor to reuse prior knowledge of how things work in the real world.
What Apple did with the calendar app is not skeumorphism: it's just kitsch. While the app looks like a calendar, it doesn't work like a calendar (you don't flip between months, for example), so it completely misses the point.
On the other hand, you could design a calendar that works like the real thing, using minimal lines and flat colors, and you could argue that it's minimalist and skeumoprhic (because it still carries assumptions about another object, the paper calendar, even thought flipping pages is not a metaphor of digital media).
BTW, the definition of skeumorphism in the Wikipedia article is too narrow, and at some points downright wrong.