WTT/WTB/WTS (want to trade/want to buy/want to sell)

PST (please send tell)

LFG (Looking for Group)

Instances (Dungeons)

Pretty much all the Instances in WoW get abbreviated too like DM (Deadmines) Stocks (Stockade) Gnomer (Gnomeregan) DS (Dragon Soul) WC (Wailing Caverns)

In general RPG terms the stats are always good places to start and then things like Party, Turn Based, ATB etc - and that people also talk about things in terms of skill levels (i.e. 525 tailoring, or maxed materia etc)

Then there's also the calling things "epic" which largely stemmed from WoW and spread into pop culture with things like Epic Fail and things that have almost obtained cult status like Leeroy Jenkins.