Yeah I think it's pretty much a combination of everything; aesthetically pleasing design (accuracy and superpseudosepiabumpmapping be damned), a nice solid noise, and an animation that makes you feel like there's power behind it. I really liked the spiker in halo 3; no reason, it wasn't that great, but it was fucking satisfying to melee someone with a gun shaped like a knife. The lancer in Gears of War or the Spartan Laser, though, both should have felt like very heavy, satisfying weapons, and both failed.
I think violating the set status quo helps too. If this is a game centered around silent sniper work, and suddenly you get a shotgun that can paint the walls with people, it feels jarring and satisfying.