According to Mathis, the crucial difference between the command line interface and the graphical user interface is that where the former is built around memorisation, the latter is built around recognition. When using a CLI, you need to memorise the various commands before you can use the computer; the advent of the GUI replaced memorisation with recognition.
Memorisation didn't disappear, though. "Memorization was relegated to shortcuts," Mathis argues, "Instead of selecting 'Duplicate' from a menu in order to create a copy of a file, people could use a keyboard command that they had memorized. But it was an optional, secondary way of achieving the same thing."
Gestures are often problematic in that they are hard to discover. My personal favourite example is the "shake to undo"-feature on the iPhone. I didn't know it existed until I read about it on the internet, and I don't know anyone 'in real life' who knows this feature exists. There is no way to find out this feature exists, other than to accidentally shake it and doing 1 + 1 = 2.
Mathis managed to dig up even better examples of incredibly non-obvious and complex gestures that you will never find out without actually diving into the documentation. For instance, you can move objects in Pages one pixel at a time by touching it with one finger, and swiping another finger into the desired direction. The gestures for matching the size of two objects is even more complicated and non-obvious.
He argues that this non-obviousness inherent to current gesture-based interfaces means they once again centre around memorisation instead of recognition. "When natural user interfaces resort to non-obvious gestures, they essentially regress into a really pretty, modern version of the quaint old command line interface," he states.
An interesting observation, and certainly one that I can agree with. I also happen to believe that despite the solutions Mathis provides us with, this lack of discoverability will ensure that gesture-based interfaces will always play second fiddle when it comes to getting real work done. As great as the iPad and Android tablets may be, they're made for consuming, not for creating.