Hallow's End

I'm just listing the quests that you need for the Hallowed Be Thy Name and meta-achievements.

Trick or Treat!
This is very easy, just talk to any Inn Keeper. You won't get a handful of candy every time. You get a debuff which keeps you from Trick or Treating more than once an hour. You'll want to collect magic wands during the duration of the event.

Out With It
Another easy task, just eat 4-6 candy until your tummy hurts :).

Bring Me The Head of ... Oh Wait.
To obtain Bring Me The Head of... Oh Wait, you need to go Brill (easiest) and get the quest then go to the Scarlet Monastery and enter The Graveyard. You'll see a small altar and pumpkin. Interact with the objects there in order to summon the Headless Horseman.

The Savior of Hallow's End
Every hour, the Headless Horseman will attack one of your faction's starting zones. You get quests from Orphan Matrons. Brill is the nearest and easiest.

That Sparking Smile
If you Trick or treat every hour, you will eventually be granted a toothpick. Use it and your done.

Rotten Hallow
You've got to go to the Hillsbrad Foothills, and complete either the Horde or Alliance seasonal quests there. There are two quests that need to be done.

G.N.E.R.D. Rage
While sporting the buff from this candy, your task will be to kill 50 players of the opposing factions. Which means you need to go to any Battlegrounds.

Check Your Head
Weighted Jack-o'-Lanterns drop from the Headless Horseman, which means killing him regularly will make your life easier. To make it easier just go to Dalaran and find the races.

The Masquerade
You'll need to find other people who have already gotten their wands, and ask them to hit you with their wands. Very time consuming.

Sinister Calling
This is the most difficult Hallow's End achievement, since both the Sinister Squashling and the Hallowed Helm drop from the Horseman. Kill him enough time and win the roll and these will be yours.

Tricks and Treats of Azeroth
Easy! Simply hit all the inns included in the sub-achievements.