WARNING: This is going to be long.
A few years ago I witnessed a gaming event that was the proverbial "straw that broke the camel's back". We were playing a 3.5 D&D game. It was a serious campaign, and we had all really gotten into our characters.
We had come upon an enemy encampment. There was a guard tower, at the top of which was a guard on watch. He had a bell which he would ring if he saw anything. So the rogue hatched his devious plan. He drank an invisibility potion, scaled the outer wall of the tower, and stealthily sneaked up beside the guard. His goal was to eliminate the guard silently so the others could approach.
The guard stood there, unaware of the rogue, looking out over the surrounding area. But what could the rogue do? His only option was a sneak attack. So he did it, inflicting a nice chunk of damage. But the guard wasn't killed, and on his next action, he rang the bell.
The player became very frustrated, and rightfully so. The guard was a mook. A nobody. A "red shirt". But as we looked over his stats, there was no way the rogue could have eliminated him silently. He was a few levels lower than the rogue, but a "one-shot-kill" was still quiet simply impossible. All that preparation and cleverness, by the rules as written, meant nothing.
The player had imagined his character as a silent blade in the night, killing his foes swiftly and silently if he could get the drop on them. Now that character concept was shattered as he realized he'd never get the chance to play that way, no matter what. The rules would never allow it. He'd never get the "one-shot-kill" against an opponent close to his own level.