Dakka

"At the point in time when bullets can pass through the interdimensional walls. When firepower takes up the entirety and eternity of space and time, all being stuck in a neverending life and death cycle as bullets recover and destroy their bodies in quick succession. No one is able to think about anything but the sheer force of the bullets rapidly flying literally everywhere in the materium turning the warp itself into nothing but a sea of semi-automatic weaponry.. Then there will be enough dakka. Or, at least almost." -The God Emperor of Mankind, If the Emperor had a Text to Speech Device, Episode 12.5[1]

*DAKKADAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKA_{go to MOAR}*
You iz dead now ya stoopid 'umie! —Every Ork ever

"Some people think they can outsmart me. Maybe... maybe. I've yet to meet one that can outsmart bullet." -Heavy Weapons Guy

"...how am I gonna stop some big mean Mother Hubbard from tearing me a structurally superfluous new behind? The answer: use a gun. And if that don't work? Use more gun." -The Engineer

Dakka is Ork slang for rapid fire capability and an onomatopoeia for the sound of automatic guns shooting. You need moar of it. No exceptions. Although other races need it too, when they use it, it's just not cool anymore. Must have something to do with that rockin' Ballistic Skill 2 that da Orks are throwing down. It is a widely corroborated scientific fact that you can NEVER have enuff dakka, because the only way that's possible is if you were to stretch an infinite plane of machine guns from one end of the observable universe to the other, and have them all firing on a bottomless magazine. And even then that's still not enuff dakka, because there are OTHER universes that still need dakka. And the Warp doesn't have enuff dakka in it. When the Materium and the Immaterium are nothing but either Shootas or bullets, that will be a good start on having enuff dakka.

Several other non-Orky races in Warhammer 40,000 have at least one weapon that relies on sheer dakka to be effective.

Imperium: Multilasers, Hellguns, Leman Russ Punisher, Lasguns, Laspistols, Heavy Bolters, Storm Bolters, Assault Cannons, Combi-bolters, Autocannons, Vulcan Megabolters, Psycannons, Heavy Stubbers, Volkite chargers and up, the amount of weapons the Adeptus Mechanicus have at their disposal would need it's own sub section.
Chaos: Harvester Guns, Mawcannons, Butcher Cannons, Storm Lasers, Reaper Auto-cannons,Hades Auto-cannons, Autoguns, Autopistols, Heavy Stubbers, Shotguns, Heavy Bolters. During the Horus Heresy era the Chaos Legions would have pretty much the same crap the Imperium had. Until the pretty boys decided they need more rape slaves.
Eldar: Scatter Lasers, Shuriken Catapults, Shuriken Cannon.
Tau: Burst Cannons, Cyclic Ion Guns, and to a lesser extent Ion Cannons
Necron: Gauss Cannons, Gauss Blasters, and Tesla Destructors.
Tyranid: Devourers, Biocannons, Deathspitters.
Dark Eldar: All sixty-seven variants of Splinter weapons, Disintegrators.

Too Much Dakka

It's actually possible to get too much dakka. In Dawn of War 2, if you pin down some Orks with enough firepower they will shout "WAAAH! Too much dakka!" In WH40k: Space Marine, the Ork Boyz will likewise shout "Zog, too much dakka!" when being cut down by heavy bolter fire, best observed in the cooperative multiplayer Exterminatus mode with several players packing heavy bolters. There's too much dakka when it's being fired at you *BLAMBLAMBLAMBLAMBLAM* HERE...HURR...UM, YOU IS SAYIN' BAD FINGS WOT YOU SHOULDN'T SAY!

Examples of maximum dakka (but not enough)

Tyranids:

HQ:

2x hive tyrant w/ 2 brain-leech worms

Elites

3x 3 hive guard w/ impaler cannon

Troops

6x 30 Devilguants

Fast attack

3x 30 gargoyles w/ fleshborers

Heavy support

3x 3 carnifex w/ 2 brainleech worms

Total dakka: 780 shots per turn

YOUZ CHOMPY GITZ SEZ THAT'S DAKKA?

Astra Militarum (Imperial Guard)

HQ: Company Command Squad 3x Leman russ punisher(1 w/ tank commander) w/heavy bolters

Troops:

6x Platoon Command Squad

6x5 Infantry Squad

6x Conscripts Squad

6x5 Heavy Weapons Team w/ Heavy Bolters

Heavy Support: 3x3 Leman Russ Punisher w/ Heavy Bolter sponsons

Total dakka: 1036, 1636 on close range or w/ orders, 2236 per turn on close range and orders. And it's without getting full platoons or even FOC. Because that would be just insane.

(but maybe enuff dakka)

"And if that don't work, use more gun."- The Engineer

MOAR

*DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKADAKKAGAKKADAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKADAKKA *DAKKADAKKADAKKADAKKADAKKAADAKKADAKKADAKKADAKKA_{go to MOAR DAKKA}*