As a few other people have said, when a knife slash is more powerful than a shotgun blast at close range, there is something definitely wrong. This is one of the major problems I have with Call of Duty, particularly with Black Ops at the moment.
Games like BF:BC2 and Halo Reach handle melee a lot better, with melee kills taking a second or two to complete and generally not being effective unless they are performed on unsuspecting targets.
Gears of War actually had a pretty good system, where melee with most weapons required multiple hits to kill, but one weapon, the lancer (which has a front-mounted chainsaw for that minority who are unaware), was able to score execution style kills.
However melee executions are not always necessarily a good thing. While playing the multiplayer trial of Alien vs Predator on both the Xbox 360 and the PS3, I found that there were moments were one player would enter the execution animation on another, only to immediately be executed by the next player, who would in turn be executed by another.
Basically, knives in COD are overpowered compared to weapons like shotguns in my opinion. Many posters on here have put ideas for how the systems could be tweaked and are well worth a read.