Put it this way, stopping power introduced to a game. Last I checked, in a fictional sense, if you've got adrenaline running and you get shot, big-whoop you keep on running. If you're running along and you catch a 5 pound wrench to the face, you're eating the pavement. It's just the difference between piercing trauma and blunt trauma. Granted the guy you just hosed should bleed to death moments after, he'd still be capable of plunging a knife to your throat. What I don't get is when you're at least 10 feet from a guy, but apparently you can close the gap like a boxer, and kill a man with a two inch blade; I call shenanigans.
I will agree on the melee being overpowered. It should be a last ditch effort if you happen to share a corner with the enemy coming the other way, but anyone with a brain inside their skull will realize that, gun in hand, bullets ftw. Knives from behind make sense, what doesn't make sense is everyone getting the lightest stuff they can find, and whatever lame perks associate it, to go around melee-ing people through shotguns and bullets. It's my opinion that if two people are in melee range and one person is shooting while the other is charging in for the kill, the guy with bullets wins. If you get the drop on someone and punch them to death, so be it. But if you make it your goal to stride the map while ignoring damage just to melee, that makes you a tool.