The way I run my campaigns, I aim for what's most realistic. (Of course, this is realism where we have orcs and magic, but realism nonetheless.)
That being said, more often than not my PC's are faced with something they feel they can't handle. This is also their astounding ability to enter situations unprepared (traveling into the underdark with 100 feet of rope between 5 people, and not a single rank in Climb in sight seems silly, agree?), but I do accept full responsibilty for it.
In my defense, I balance the CR's as best I can with their average level. To their credit, they've often surprised me.
:\ Unfortunately, I seem to kill one of my players more than any of the others, but I think this is because she has horrible luck. She always gets herself into the wrong place, at exactly the wrong time.
My current campaign, a homebrew, the death toll is up to 7. More, if you count the party NPC's lost. (Though, it's not like THEY do, the bastards.)