Why doesn't my MacBook Air wake up from sleep when I touch the trackpad?
In Mac OS X 10.7, aka Lion, Apple removed the ability to wake the Mac by moving the mouse. This stopped knocks to the table and pets moving the mouse accidentally from waking the Mac.

By clicking or pressing a key, you are being more deliberate about your intention to wake your Mac. Ideally it should help avoid waking your Mac accidentally and save a little power.

As ever, this improvement benefits some users more than others.