I should point out though that while there may be genuine omissions from this list that can be added later, in general, this list is not negotiable...this means that it is not an excuse to bypass the 'no netspeak' rule that we have here on VTM. Please bear that in mind.
Allowed HP Abbreviations
VTM - Veritaserum!!
HP - Harry Potter
RW/R - Ron Weasley
Hr/HG - Hermione Granger
GW - Ginny Weasley
SS - Severus Snape
McG - Mcgonagal
DD - Dumbledore
DE - Death Eaters
LV - Lord Voldemort
PS/SS - Sorcerors/Philosophers Stone
CoS - Chamber of Secrets
PoA - Prisoner of Azkaban
GoF - Goblet of Fire
OotP - Order of the Phoenix
HBP - Half-Blood Prince
JKR - Joanne Kathleen Rowling
QWC - Quidditch World Cup
WB - Warner Brothers
MWPP - Moony, Wormtail, Padfoot and Prongs, AKA The Marauders
DADA - Defence against the Dark Arts
MoM - Minister of Magic / Ministry of Magic (look at context)
DoM - Department of Mysteries
RoR - Room of Requirement
CoMC - Care of Magical Creatures
OBHWF - One Big Happy Weasley Family
AK - Avada Kedavra
Allowed Abbreviations - General
OMG - Oh My God
IMHO - In My Humble/Honest Opinion
LMAO - Laugh my *bleep* off
LOL - Laugh Out Loud
ROFL - Rolls on floor laughing
(sp?) - Spelling? i.e. unsure that this is spelled correctly
PM - Private Message, or Owl
BTW - By the Way
HP terms - General
Harmonian - Someone who supports the HP/HG relationship, or 'Ship' for short. (For an explanation of why this 'ship' is also called 'HMS Pumpkin Pie', read this.
Heron - Someone who supports the RW/HG ship.
Chocolateer - Someone who supports the HP/GW ship
Canon - Something considered fact as coming from the books themselves or one of JKR's interviews. For a fuller explanation, see Bajab's explanation here.
If there's anything not included here that is commonly used in HP fandom and you think it would help newcomers out, then please contact a staff member via Owl.
Thanks, and I hope you found this helpful!