This has several ramifications. First, Apple's added a new click gesture called the Force click--which is what happens when you click and then push a little bit harder, until you feel a second click. Apple's built in force-click actions to many of its apps, including the Finder (it opens a Quick Look window), and other developers can choose to support it too.
Developers also have the ability to access the haptics in the trackpad to provide another dimension of interface feedback. Apple can vibrate the trackpad to provide extra feedback--for example, imagine an app letting you know that the object you're dragging has reached the center of the document by giving you a brief bump on the trackpad. It will be interesting to see experimentation with this new piece of hardware.
But the bottom line is, this is a trackpad, and it feels like one--plus it's programmable.
One of the biggest compromises Apple made in designing the MacBook to be as thin and light as possible was to create a new, thinner keyboard. In order to make the keyboard thinner, Apple reduced the amount of key travel--the amount of space that the keys move when you press them.
As someone who types for a living, and who types roughly 115 words per minute, this is a huge change. The reduced key travel is instantly noticeable--there's just much less physical feedback as you press each individual key. It feels like a cross between typing on a more traditional Mac keyboard and tapping on the hard glass screen of an iPad. (No travel at all there!)
Apple seems to have realize that the reduced travel has made this keyboard less appealing, and has attempted to offset the change with a bunch of other changes that improve the typing experience. There's a new butterfly key mechanism atop stainless steel dome switches, which Apple says increases key stability, and the keys are all a bit wider than on a traditional keyboard, so there's more area to hit on each key.
These changes help, but they don't really offset the reduced travel. The MacBook keyboard's better than I expected it to be--I was able to score 118 words per minute on TypeRacer using it--but it never felt particularly comfortable. If you're not a keyboard snob, you may not even notice the difference, but if there's any single feature that would make me reluctant to buy a MacBook, it would be the keyboard.
Beyond the changes to the key movement itself, this keyboard offers a few other interesting features. Each key is individually LED backlit, which is supposed to reduce light leakage, but I found the lighting of the key labels not to be uniform. Edges of several key labels (the left side of the Esc key, the bottom of the delete and tab keys) were darker, as if they weren't properly lit.
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