WoWDB

Overview

Headless Horseman is a fight with three phases but in practical terms it only has two - tank'n'spank and burn. The encounter tests players on switching targets and straight-up DPS. Since Cataclysm, the only means of accessing the boss is queuing either through the Looking For Dungeon tool or speaking to Headless Ghost in any faction and sanctuary capital city (excluding Pandaria). A player can kill HH as many times as he or she desires for a chance to win loot from his corpse but will only be awarded  Loot-Filled Pumpkin for the first kill of the day. The boss fight is initiated by interacting with Pumpkin Shrine. 

Notable loot from the loot-filled pumpkin (the last two items are not exclusive to it):

•  The Horseman's Reins
•  Sinister Squashling
•  Hallowed Helm
•  Magic Broom

Notable loot from Headless Horseman (2012 & 2013):

•  Band of the Petrified Pumpkin
•  The Horseman's Horrific Hood
•  The Horseman's Ring
•  The Horseman's Sinister Slicer (very nice weapon for transmogrification)
•  Seal of Ghoulish Glee
•  Wicked Witch's Signet

Notes

Headless Horseman exists in a special version of Scarlet Monastery: Graveyard which may only be accessed via Looking For Dungeon. Of interesting note is that it is the old (pre-MoP) version of the instance. Players will need to be at least level 89 to queue for the holiday boss encounter. It is possible for characters lower than level 89 to get around this restriction by having five friends or willing PuG players create a party, queue for Headless Horseman, and enter the dungeon. One of the DPS party members must drop from the instance group. The party leader then invites the lower-level character to group and that character accepts and joins the group. The lowbie then teleports in to the dungeon via Teleport to Dungeon option (click the LFD tool icon near the minimap). Please note: the lowbie must meet the minimum level requirement for the normal version of the dungeon, so he or she must be level 23+; and he or she will not receive the  Loot-Filled Pumpkin.

Strategy

Phase 1

The tank should immediately Taunt when the boss lands on the ground. DO NOT BLOODLUST. DPSers attack the Headless Horsemen - from behind or at range so they aren't being hit by  Horseman's Cleave - down to 1%. At 1% HP, the head will separate itself from the body. DPS the head until it rejoins the body (a target macro may be useful) at ≈66% HP or when the body finishes healing back to 100%, whichever comes first. Meanwhile, watch out for the body's Whirlwind attack.

Phase 2

The tank should immediately Taunt the boss again once he's reformed. This phase is just like Phase 1 but with the addition of HH's  Conflagration ability, which causes the target player to burn and be disoriented. Players will need to take care to keep distance between themselves because the conflagrate deals damage to nearby allies. Healers need to actively heal the conflagrated player because the spell's damage is percentage-based. (Players with abilities which make them Immune can cleanse the debuff from themselves, ex: mage's  Ice Block.) Tanks will never be targeted by Conflagration but everyone else is fair game.

DPS the boss to 1% and the head separates. The head will rejoin the body at ≈33% HP or when the body finishes healing back to 100%.

You can choose to Bloodlust for this phase or wait until the final phase depending on how well the dungeon group is doing in the encounter. I personally like using BL at this point because Conflagration is annoying.

Phase 3

The tank should immediately Taunt the boss yet again. HH will not use Conflagration in this phase. Instead, he uses a different ability to summon four Pulsing Pumpkins. After ≈5 seconds, they "hatch" into Pumpkin Fiends. Every 30 seconds afterwards, he will summon four more of the adds. The tank should be ready to pick up the adds so they can more readily be killed with DPS cleave, but the adds can otherwise be ignored. Burn the boss's HP down.

If you haven't used Bloodlust yet, go ahead and cast it.

Enjoy your loot. :)