Before Mac OS X 10.9 Mavericks, pressing the power button on a Mac triggered this dialog box. A very easy-to-remember shortcut for a very useful dialog box. In Mavericks, Apple changed the behaviour of the power button, in a really arbitrary fashion. From a usability standpoint, I think it’s generally a bad idea to suddenly change a shortcut that has remained unchanged for years. This power button shortcut has existed — if I remember correctly — since Mac OS 8. Long-time Mac users have it in their muscle memory.
In Mavericks, or more specifically, in Mac OS X 10.9.0 and 10.9.1, the new behaviour works like this: tapping the power button once puts the Mac to sleep. Tapping the power button again wakes the Mac up. If you want to access the familiar Shut Down dialog box, you have to press the power button for 1.5 seconds (according to this Apple Support article, which hasn’t been updated yet). If you hold down the power button for at least 5 seconds, you’ll force the Mac to turn off.
Again, I didn’t see the practical side of this change. Some people may have found it handy, but I really think it was unnecessary. In more than one occasion I accidentally put my MacBook Pro to sleep when I was actually trying to quickly access the Restart option from that dialog box.
The recent 10.9.2 update changes the power button behaviour again. Now, if you just tap the power button, nothing happens. If you press it for a couple of seconds, you put the Mac to sleep, and if you want to access the Shut Down dialog box, you have to press Ctrl-power button. This is getting confusing and ridiculous, a textbook case of ‘Don’t fix what is not broken.’