In Fallout 3, if a killing shot is a limb or the head, there is a 50% chance it will be severed. If it does not fly off, then the game does a check for the limb exploding, which again has a 50% chance. Thus you've got a 75% chance that some kind of dismemberment will happen.
At low levels when you have bad aiming skills that's not too over-the-top, as your chances to hit someone's head is quite low, and thus it is nice to get a gory reward when such a shot is succesful and it is a kill shot. But at high levels, when you have a 95% chance to hit, it gets a bit over the top.
Some weapons are set to slice-off only, and some are set to explode only, and thus have just a 50% chance of some sort of dismemberment happening.
There are a couple of Fallout 3 mods that allow you to increase or decrease these percentages. Decreasing each chance to 25% works nicely. Annoyingly, you can't change the percent chance of dismemberment on a per-weapon basis, they are global values.
One thing I tried was modding most weapons to be explode only, and lowering the chance to 25%. The only weapons that still had a chance to sever were lasers and bladed weapons, and I upped that percentage to 75%. While I can't have my good old classic Fallout laser torso-slice, at least I can have lots of severed legs and decapitations!