This is going to sound a little weird: one of the worst things that can happen when you hit the power button is that the computer turns off.
Why is that bad? Well, it's bad if you're in the middle of work and hit the power button accidentally. Any unsaved work can subsequently vanish.
But isn't the system supposed to shut down when you press the power button? Well, maybe. It all depends. For example, if you have kids who like to push all buttons within eyesight (as kids do), or if your power button is imperiled by your leg, you can configure what happens when it's pressed. Here's what to do:
This is great for families with small kids. I tell my son, for example, that all he needs to do when leaving the computer is to hit the power button. The system hibernates, and neither of us is ever in danger of losing valuable data.