Zed-time: In certain situations, showing you just how awesome something you just did was.
Example: Using a grenade launcher on a stalker and listening to it shriek in slow-mo.
The pump-action-shotgun deals these out constantly. Watch the Zed's head explode into hundreds of little pieces as you slooowly pump the shotgun.
This is quite frankly the best weapon for it.
Zed-time extensions generally involve a whole lot of awesome, as you're mostly either nailing zeds left and right with assault/battle rifles or cutting them down one after the other with melee weapons.
However, watching yourself reload a gun in slow motion isn't that great.
The Patriarch. Come on, having a chaingun and a rocket launcher for an arm? How is that not awesome? How 'bout the big-ass tentacle sticking out of his chest?
It gets better. It's hard to tell if it's happening to you, but when he gets close, that tentacle is his melee attack, which can result in you being Punched Across the Room.
The .50 Caliber rifle. Nothing as awesome as tapping the Patriarch in the back of the head as he runs away (invisible, mind you), sending him flying arse over bollocks.
Finishing a tough round as the Sole Survivor, especially if it's saving the whole match by facing down the Patriarch alone.
The finale to Fright Yard in Objectives Mode. After bending over backwards for that asshole Halliday, he tries to kill you with a Patriarch AND the added pressure of his toxic mortar cannon. Then when you finally kill the Patriarch, you pack a small motorboat with dynamite and send that bastard straight to Hell!!
That first time you calmly remove an enraged Fleshpound's head with the crossbow while the rest of your team panic fires.
Stun-locking a Scrake to death as a berserker.
Even better, melee locking the Patriarch and turning muscle memory into the best revenge for every time the Patriarch killed you. Bonus points if done in a team game on Hell on Earth.