Well, while this is bumped, I'll contribute some BC stuff:
Misc.
T4/T5/T6 - Tier 4/5/6. Refers to level of gear, or level of raid. Tier 4 is entry level raiding, followed by Tier 5, followed by Tier 6.
S1/S2/S3 - Season 1/2/3. Refers to Arena Seasons, often used to designate gear (i.e. S1 Shoulders referring to the Arena Season 1 shoulder armor).
Gear/Titles are as follows
Season 1 - Gladiator's...
Season 2 - Merciless Gladiator's...
Season 3 - Vengeful Gladiator's...
Zones
HFP - Hellfire Peninsula
Zang - Zangarmarsh
BEM - Blade's Edge Mountains
SMV - Shadowmoon Valley
Instances/Raids
H - Heroic Mode: Refers to Dungeon Difficulty Setting. Heroic Mode instances have their mobs, bosses, and loot re-tuned to level 70 players. All 5-man Burning Crusade instances have a heroic mode, accessible once a player reaches 'Honored' reputation with the instance's faction and purchases the corresponding heroic key.
ex. H Mech refers to The Mechanar on Heroic Mode.
Old World
Kara - Karazhan: 10-man T4 raid located in Deadwind Pass.
ZA - Zul'Aman: 10-man T4/T5 level raid located in Ghostlands.
Caverns of Time (Tanaris)
Durn - Escape from Durnhold Keep: level ~66-68 5-man.
BM - Opening the Dark Portal/Black Morass: level ~68-70 5-man.
MH - Battle of Mount Hyjal: level 70 25-man T6 raid.
Outland
Hellfire Citadel (HFP)
Ramps - Hellfire Ramparts: level ~60-62 5-man.
BF - Blood Furnace: level ~61-63 5-man.
SH - Shattered Halls: level ~69-70 5-man.
Mag - Magtheridon's Lair: level 70 25-man T4 raid.
Coilfang Reservoirs (Zang)
SP - The Slave Pens: level ~62-64 5-man.
UB - The Underbog: level ~63-65 5-man.
SV - The Steamvault: level ~70 5-man.
SSC - Serpentshring Cavern: level 70 25-man T5 raid.
Auchindoun (Terokkar Forest)
MT - Mana-Tombs: level ~64-66 5-man.
Crypts - Auchenei Crypts: level ~65-67 5-man.
SH - Sethekk Halls: level ~67-79 5-man.
SLabs - Shadow Labyrinth: level ~69-70 5-man.
Tempest Keep (Netherstorm)
Mech - The Mechanar: level 70 5-man.
Bot - The Botanica: level 70 5-man.
Arc - The Arcatraz: level 70 5-man.
TK - The Eye: level 70 25-man T5 raid.
Other
Gruul's - Gruul's Lair: level 70 25-man T4 raid. Located in BEM.
BT - Black Temple: level 70 25-man T6 raid. Located in SMV.
(I'll add more later, this is just what I came up with off the top of my head before I had to run off to school)
Edited, Jan 17th 2008 7:47am by Placeholder