Epic
Other than legendary, the rarest and highest quality type of equipment available
EotS
Eye of the Storm, a 15-man battleground with four towers for controlling and a flag in the middle for capturing.
Durn
Old Hillsbrad Foothills, a Caverns of Time instance in Tanaris. Refers to Durnholde Keep, which you must escape from in the instance.
DPS
Damage Per Second
DoT
Damage over time. This refers to spells that cause damages in multiple ticks over a period of time
DM
Dire Maul, an instance in Ferelas. Sometimes means Deadmines, an instance in Westfall
Debuff
The opposite of a buff, usually a spell that lowers the target's attributes. Debuffs can sometimes be dispelled by certain spells or abilities.
DE
Disenchant, create enchanting materials from uncommon, rare, or epic items.
DD
Direct Damage. This refers to spells or abilities which deal all of their damage on impact, rather than over time.
Darnassus
Darnassus, home of the Night Elves