IO540, I think it's even simpler. I think all the pilots on here will not take-off in an airplane while knowing that a certain bit of essential kit is inop. Of course, what one considers essential may vary a bit from pilot to pilot, from flight to flight, and the POH has a few things to say on that as well. And some of us have maybe skipped an item or two (pitot cover anyone?) on the pre-flight check and come to regret that afterwards.
What does differ is the amount of drama we make when discovering something is just not right, in-flight. There's one camp that will make an informed decision about the consequences of the failure and continue or abort the flight as necessary, flying the airplane by feel if necessary, and tell the engineers afterwards. And there's a camp that treats every failure as a mayday scenario, and once safely on the ground they tell the people propping up the bar first, and the engineers later.
I guess one of the differences between the two camps is the amount of training they've had, and the experience they've accumulated. If you've never been trained to land an aircraft without instruments whatsoever, then every instrument failure is an emergency. Just like if you've never been trained to read a map, your GPS failure will constitute an emergency.
