There's one small change that comes with OSX 10.7 (Lion) that I'm finding fascinating from a user interface perspective: the reversal of scrolling direction with the touchpad. By default in Lion, when you move your fingers down, your view goes to the top of the page, and when you move your fingers up, your view goes to the bottom of the page.
At first, this feels incredibly awkward, and I was thankful that there was a setting to reverse everything back to normal. I mean, of course when I move my fingers down on the touchpad, I want to go further down the page, right? Why would Apple dare change something so obvious and intuitive?
But wait a minute... the obvious way wasn't the way I scrolled on a touch surface like the iPhone or iPad and I didn't remember those feeling awkward. So I decided to experiment with leaving the reverse (back to "normal") setting off.
A few days later, and now the crazy new way of scrolling feels obvious. I suspected that it might, that it just felt odd because I was used to doing things the other way. But I didn't expect to be able to switch back and forth between old and new easily, which is what happened. I now hardly notice the change from a 10.6 default touchpad scrolling style to the 10.7 default style.
Why is that?
Well, the best I can reason is that both are pretty decent mental models. To make the "old" way feel natural, you simply imagine that the portion of a page that you see represents your field of vision, and to see more beyond the last point at the bottom, you need to move your "eyes" (via the touchpad) down. To make the "new" way feel natural, you move the virtual page instead of your virtual field of vision. So, just as you would read the bottom of a real page if you couldn't move your eyes, you push up on the "page" (via the touchpad).
You could make an argument that the "page" model is the more accurate one, as you can actually see other parts of the screen (and thus the page isn't really just out of your field of vision, but rather it's completely obscured by other elements on the page or the bottom of your monitor). If you think of it, it seems like a lot less work to push one element up in order to see the bottom of it, as opposed to pushing all the other elements down. And that's effectively what you're doing with the "old" style of scrolling.
It will be interesting to see if Apple manages to push overall change in this area. I imagine they're going to have a few things going against them: (1) the option they provided to switch back is probably easier than forcing your brain to recalibrate its expectations, (2) most people will probably mistake "habit" (or "tradition") for "natural" (it's not like we don't do this in other areas of life), and (3) the touch interface isn't directly connected to the display like it is with an iPhone or iPad. I think the last of these is why this "new" method feels odd in the first place. When the touch interface is the display, we have to construct even less of a mental model. Most of it's just there for us. With a touchpad and regular monitor, our brain has to do one extra translation step, and that's just enough to make it a challenge to switch models.
To be clear, I think both ways are completely valid and don't think that one provides a huge advantage over the other when the touch interface and viewing interface are disconnected as with a laptop or desktop. But I think it raises an important question that should be on the mind of anyone designing user interfaces: are you adapting the machine to the person, or are you counting on their ability to adapt to the machine? The former can feel frustrating and nitpicky and far more trouble than it's worth, but after you get it right, it's hard to imagine settling for the latter.