Mind Flay is a channeled spell that can be acquired from the Shadow talent tree. It is the spell with the best damage/mana ratio for the priest, and it has the additional bonus to slow runners by 50%.
Mind Flay used to receive a largely reduced amount of spell damage (59% of total +spell damage, divided over 3 sec.). Originally, Blizzard said this was due to its snare. Later they revised their statement, claiming that it would be too powerful.
Its debuff has no classification, hence it cannot be dispelled.
When attacking a Druid with Mind Flay, shapeshifting cancels it.
Similarly, if the target uses a PvP trinket, it removes Mind Flay, due to the snare.
Mind Flay does additional threat. This is most likely due to its snare.
This spell is popularly referred to as the "face melter" due to the ray that usually targets the opponent's face.
Patch changes
Patch 3.3.0 (2009-12-08): The range of this ability has been increased to 30 yards, up from 20.
Patch 3.0.8 (2009-01-20): Fixed a bug with targeting where you would not deal damage if not facing the target while channeling. Also corrected an issue where the damage from this ability was slightly delayed.
Patch 3.0.2 (2008-10-14): Can now periodically critically hit. The spell coefficient has been increased approximately 30%.
Patch 3.3.0 (2009-12-08): The range of this ability has been increased to 30 yards, up from 20. 
