Attention to detail is one of Apple's hallmarks, and the user interface is one area where Apple has been consistently ahead of the curve. From the rendering of text to the animation of almost every on-screen element be it a new application opening or the click-and-drag motion of file management. It has always been a noticeable contrast to the Windows OS which for most of it's life popped things in and out of existence before your eyes without much mind to these little details. This is changing, as anyone with experience of the Windows Aero interface will have noticed. When you interact with objects in the taskbar, for example, the preview box is beautifully animated and Aero Peek fades windows gracefully from view. Nowhere is this shift towards presentation more apparent than in mobile devices such as smartphones and tablets. To me, it represents something more fundamental than just the spread of a more aesthetically pleasing user interface paradigm (aesthetics are a key feature of a good interface) but it also represents a fairly significant shift in the nature of computing and software design.
When I studied computing, the system came first. The user experience was at the bottom of the priorities list for allocating system resources. Consider that at the time I was studying, the 1GHz CPU was a big deal, the first computer I built had a 233MHz processor! You simply couldn't give up that much processor to providing smooth animations and expect the real work of the machine to get done. You had to prioritise the task and not the person waiting for it to complete. Now we're in the age of multi-core, hyperthreaded CPUs running at over 2GHZ things are different. We're in a position where we can dedicate resources to providing an interface that is beautiful and smooth and be certain that we're not going to hinder the machine's essential processes.
The device that really brings this paradigm shift to light for me is the Asus Nexus 7. Android never felt smooth to me in the past, but I recently got to use a Nexus 7 with Jellybean for a few minutes and it's incredible how well animated and responsive the interface feels. The OS does it's damnedest to provide a consistent 60 frames per second and it makes it a joy to use at no noticeable detriment to the performance of apps running on the device. To me, this is a step along the road of changing the way we interact with our computers; where the interface should feel as natural as reading a book, riding a bike, making a coffee or asking someone a question. The less you are reminded that there's a computer (as we used to understand them) involved, the better.