You know, UIs that look like their physical counterparts - iBooks, or apps that use metaphors of real world physical objects - Twitter for iPad, all stuff highly popularised by Apple both on iOS and now on the Desktop.
My favourite application of this concept is perhaps the iPhone, mainly because bringing the metaphor to a full circle would make a huge impact on this type of device.
The Missing Pieces
In Bill Buxton's "[Sketching User Experiences](http://www.amazon.co.uk /Sketching-User-Experiences-Technologies-ebook/dp/B001GS3P9S)" book, right in the beggining, in a chapter called 'Design for the Wild', he introduces us to 3D tech from the 1880's, the 'Ammassalik wooden maps'.
These objects are easy to use, intuitive, can get wet, dropped, kicked, stored, and will always be there, fulfilling their main purpose, whenever needed.
That doesn't happen with anything digital, no matter how well designed, our phones, laptops, fire alarms, all exist only while there's electricity to power them, as long as they don't get wet, you get the idea. You probably don't notice it anymore, it may have become a reflection to some to be extra careful with their phones, the same way a person will try and rest his hand or put something away in his pocket even while naked (true story).
Yet another problem technology has yet to solve, how to prevent our magical devices from becoming high tech bricks after a few hours of use. This one's particularly amusing, a sick joke played on us, here we have all these marvellous devices, but, like an electricity junkie robot Cinderella, they need to jacked in to the grid for their regular fix of electrically charged particle dough.
I've talked about durability, availability, but there's still one other aspect that's missing in our shiny tangible interfaces, that is the ability to use them as the Ammassalik use their maps while out in the cold and harsh sea: tactile recognition.
We can resort to muscular memory to some extent, but a smooth screen can't offer reference points such as the little bumps of the F and J keys on your keyboard. Interestingly enough, the iOS keyboard on the iPad has a visual representation of these, although completely absent of functionality and is probably just a reflection of the approach behind the design of the keyboard (special note to the fact the designers tried to mimmic a real keyboard but still took advantage of the digital nature and implemented features such as the split keyboard/touch&drag for quick input of numbers&symbols).
For blind people the advantages of tactile interfaces would be obvious, but beyond that I can't even imagine how different must it be to use a device like a touch screen phone where you can feel the textures in the application icons and the borders of the icons themselves, feel a search box, how would it affect the UI metaphor (would a search box, inputs, still be inset into the UI?)
In The End
Here's hoping that one day not too far away we'll find out.
Almost positive Apple will be among the first ones to successfully push such devices into the mass market, leading to another mass misinterpretation of the metaphor, if you hate over the top text shadows and the like, imagine a device where the buttons start squeaking as the device gets older just because it's what (low quality) hardware does in real world.
Or things like this: [Nokia Kinetic](http://thisismynext.com/2011/10/27/nokia- kinetic-device-nokia-world-2011-demo/), alien device featuring a rather cool technology, imagine if they had instead created an eBook app where you could fold a corner to toggle page bookmark.