This has all the abbreviations from N-Z alphabetically. You'll find A-M here. The categorized list is this way, or that way for class-specific abbreviations. Please leave comments on the main post.

Naxx: Naxxramas [raid]
NE: Night Elf
Nerf: A change to the game that makes a given class/spec weaker
NF: Nightfall [warlock]
NPC: Non-player character
NR: Nature resistance
NS: Nature's Swiftness [druid]
NS: Nature's Swiftness [shaman]
NS: Netherstorm [zone]
NW: Netherweave
Ofc: Of course [chat]
OH: Off-hand
OL: Outlands [zone]
OP: Original post/original poster [forums]
OP: Overpowered
OMW: On my way [chat]
Ony: Onyxia [raid]
OoC: Omen of Clarity [druid]
OOM: Out of mana
OP: Overpower [warrior]
Org: Orgrimmar [zone]
OT: Off-topic [forums]
Pally: Paladin [paladin]
Pat: Patrol
Paw: Mark/Gift of the Wild [druid]
PBAoE: Point-blank AoE (an AoE spell that radiates out from the caster, like Arcane Explosion)
PI: Power Infusion [priest]
PoH: Prayer of Healing [priest]
Poly: Polymorph [mage]
PoM: Prayer of Mending [priest]
PoM: Presence of Mind [mage]
Port: Portal [mage]
Pot: Potion
Premed: Premeditation [rogue]
Prep: Preparation [rogue]
Proc: As best as I've been able to figure out, this is short for "Process" "Procedure" (see WoWWiki). What it refers to is things that have a chance to go off when some event happens. For instance, a Fiery enchant has a chance per hit to cause extra fire damage. This ability of the Fiery enchant is known as a "proc;" when it activates and causes the extra damage, one can say that it has "procced," and the activation itself, the extra damage, can also be called a "proc."
Prot: Protection talent tree [paladin]
PS/Fear: Psychic Scream [priest]
PST: Please send tell [chat]
PSW: Please send whisper [chat]
PUG: Pick-up group (a group formed "on the spot" by use of the LFG tool or a channel or something, as opposed to a guild group or some other group of friends)
PvE: Player vs. environment
PvP: Player vs. player
PW:F/Fort: Power Word: Fortitude [priest]
PW:S/Shield: Power Word: Shield [priest]
Pyro: Pyroblast [mage]
QFE: Quoted for excellence [forums]
QFT: Quoted for truth [forums]
QQ: Doesn't "stand for" anything per se; rather, it's an emoticon: it looks like two crying eyes. It basically means "you're complaining," although it can also be used as a verb ("stop QQing"). [forums]
RAP: Ranged attack power
Res/Rez: Resurrect/Resurrection
Resto: Restoration talent tree [druid]
Resto: Restoration talent tree [shaman]
Ret: Retribution talent tree [paladin]
RFC: Ragefire Chasm [instance]
RFD: Razorfen Downs [instance]
RFK: Razorfen Kraul [instance]
RL: Real life
Roots: Entangling Roots [druid]
RP: Role-play[ing]
Sac: Demonic Sacrifice [warlock]
Salv: Blessing of Salvation [paladin]
SB: Shadow Bolt [warlock]
SB: Shield Bash/Shield Block [warrior]
SC Strat: Scarlet side of Stratholme [instance]
Scholo: Scholomance [instance]
SF: Seal Fate [rogue]
SF: Shadowform [priest]
SFK: Shadowfang Keep [instance]
SH/Shat: Shattrath City [zone]
SH: Shattered Halls [instance]
Shammy: Shaman [shaman]
Sheep: Polymorph [mage]
ShS: Shadowstep [rogue]
SL: Shadow Labyrinth [instance]
SL: Siphon Life [warlock]
SL: Soul Link [warlock]
SM: Scarlet Monestary [instance]
SM: Shadow Mastery [warlock]
SMC: Silvermoon City [zone]
SMV: Shadowmoon Valley [zone]
SnD: Slice and Dice [rogue]
SoB: Seal of Blood [paladin]
SoC: Seal of Command [paladin]
SoC: Seed of Corruption [warlock]
SoE: Strength of Earth totem [shaman]
SoL: Seal of Light [paladin]
SoL: Surge of Light [priest]
SoR: Seal of Righteousness [paladin]
SoR: Spirit of Redemption [priest]
SoS: Swamp of Sorrows [zone]
SotC: Seal of the Crusader [paladin]
SoV: Seal of Vengeance [paladin]
SoW: Seal of Wisdom [paladin]
SP: Slave Pens [instance]
Spec: Specialization (in a profession or talents)
SR: Shadow resistance
SR: Silent Resolve [priest]
SS: Shadowstep [rogue]
SS: Sinister Strike [rogue]
SS: Soulstone [warlock]
SS: Steady Shot [hunter]
SS: Stormstrike [shaman]
SS: Sweeping Strikes/Shield Slam [warrior]
ST: Temple of Atal'Hakkar (a.k.a. Sunken Temple) [instance]
STFU: Shut the f--- up [chat]
STM: Stonetalon Mountain [zone]
Stocks: Stockade [instance]
STP: Stonetalon Peak [zone]
Strat: Stratholme [instance]
STV: Stranglethorn Vale [zone]
Succy: Succubus [warlock]
SV: Steamvaults [instance]
SW: Shield Wall [warrior]
SW: Stormwind City [zone]
SW:D: Shadow Word: Death [priest]
SW:P: Shadow Word: Pain [priest]
T1/T2/etc.: Class-specific "tier" sets of gear that drop in raids.
Tap: Life tap [warlock]
TB: Thunder Bluff [zone]
TBH: To be honest [chat]
TC: Thunder Clap [warrior]
Tele: Teleport [mage]
Tero: Terokkar [zone]
TF: Tiger's Fury [druid]
TL;DR: Too long; didn't read [forums]
TM: Tactical Mastery [warrior]
TM: Tarren Mill [zone]
TM: Thrallmar [zone]
ToL: Tree of Life [druid]
ToW: Totem of Wrath [shaman]
TP: Teleport [mage]
TSA: Trueshot Aura [hunter]
UA: Unstable Affliction [warlock]
UB: Underbog [instance]
UBRS: Upper Bloackrock Spire [instance]
UC: Undercity [zone]
UD Strat: Undead side of Stratholme [instance]
UD: Undead
UI: User interface
Ulda: Uldaman [instance]
VC: Deadmines [instance]
VE: Vampiric Embrace [priest]
VT: Vampiric Touch [priest]
VW: Voidwalker [warlock]
Warr: Warrior [warrior]
WC: Wailing Caverns [instance]
WC: Wrong channel [chat]
WF: Windfury (weapon or totem) [shaman]
Wis: Blessing of Wisdom [paladin]
WPL: Western Plaguelands [zone]
WSG: Warsong Gulch [battleground]
WTB: want to buy [chat]
WTF: What the f--- [chat]
WTH: What the hell [chat]
WTS: Want to sell [chat]
WTT: Want to trade [chat]
WW: Whirlwind [warrior]
XR: Crossroads [zone]
Zerker: Berserker stance [warrior]
ZF: Zul Farrak [instance]
ZG: Zul'Gurub [raid]
ZM: Zangarmarsh [zone]
ZS: Berserker Stance [warrior]

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