This year's incarnation of Hallow's End is notable for arriving less than a week after patch 4.0.1, which disheartened a lot of players who had been expecting to complete the yearlong meta as the holiday arrived and felt cheated out of free 310 percent flight. Happily, Blizzard's decided to continue awarding the 310 percent flight proto-drake for the achievement, so if you nab What A Long, Strange Trip It's Been this Hallow's End but didn't already have a super-fast flyer, you should be grandfathered into the 310 percent flight skill without having to pay for it. Hallow's End is also subject to the new rules implemented this past year requiring players to queue for holiday bosses through the dungeon finder.
You'll find a full guide to the holiday achievements past the cut!
This is very easy to do at the beginning of the holiday when a lot of players can help put out village fires, and less easy to do as the holiday winds on and fewer people get involved (it's similar to This One Time, When I Was Drunk from Brewfest in that respect).
NPCs in major cities will direct you to one of your faction's level 10 villages: Goldshire, Kharanos, Azure Watch or Dolanaar for the Alliance, and Brill, Razor Hill, Falconwing Square or Bloodhoof Village for the Horde. Each is regularly attacked by the Headless Horseman, who sets several buildings on fire. Your task is to talk to the visiting Orphan Matron and then help the locals by dousing the flames. How do you do this? You'll be given a bucket, and in each town there'll be a centrally-located water barrel. Hotkey the bucket, right-click it in proximity to the barrel in order to fill it, run anywhere there's a fire, and use the bucket. You'll get a targeting circle which you can aim at the flames. Lather, rinse and repeat until all the fires are out; it's really pretty simple.
I would try to do this achievement as early as possible when lots of people are involved with the holiday, but if you don't have any choice but to do it alone, head for Azure Watch (if you're Alliance) or Falconwing Square (if you're Horde). It's considerably easier in both towns to solo the achievement, but early in the holiday it's almost impossible not to get it with so many people putting out fires.
Trick or Treat!
As the holiday goes live, big candy buckets suddenly appear in inns all around Azeroth. Right-clicking one will give you a handful of candy with random sweets granting small buffs:
Hallowed Be Thy Name meta, and you'll get it anyway while doing Tricks and Treats of Azeroth (see below).
Set aside any G.N.E.R.D.S. you get here and keep moving!
Killing the Headless Horseman is pretty easy, and it'll be even easier this year now that you'll queue for a kill through the dungeon finder. We haven't heard anything about an updated loot list, so I would assume that Hallow's End is in the same situation as Brewfest 2010; the boss should go down at record speed but won't reward anything new. We're in the run-up to Cataclysm, and I doubt the developers have any spare time left from killing themselves trying to get the next expansion out to update holidays. We might see an updated Winter's Veil but probably won't see any new holiday-related stuff in the meantime.
Otherwise, this should be easy. Pull up your dungeon finder (the default hotkey is "I") and the Headless Horseman should appear in the dungeon selection options. Queue for a group (or just queue your existing group), and you'll be ported as soon as possible. If you've never done the Headless Horseman before, he's pretty easy, and you can find a strategy guide here.
That Sparkling Smile
This one's easy enough, but you'll have to suffer through a little RNG for it. In addition to picking up sweets from the candy buckets located in inns, you can also "trick or treat" with innkeepers. Just talk to them; it'll be a new option in their conversation box. Innkeepers may decide to trick you by turning you into one of several creatures for a minute, but they'll usually give you a treat bag. Tooth picks are a potential reward from these bags, so just keep trick or treating and a few should eventually show up. The only catch is that you can't trick-or-treat with innkeepers more than once per hour, so you'll need to revisit them on a set schedule.
Treat bags can otherwise contain an assortment of candies, Hallow's End masks, magic wands (see below for another achievement), Hallowed Helms or even (very rarely) a Sinister Squashling, so you've got more reasons to keep chatting with innkeepers.
This one's pretty straightforward as long as you've got a few extra G.N.E.R.D.S. hanging around, although having to do it post-patch with damage off the charts is painful. Queue for a battleground (Alterac Valley or Isle of Conquest are your best bets) or Wintergrasp, eat a G.N.E.R.D. once you zone in, and stay with a pack of players as much as you can.
You don't have to get all 50 honorable kills in one battleground, but I found this to be an easy achievement to do in Alterac Valley on the bridge chokepoint. Isle of Conquest during pitched battles over keep doors was equally fast on another character. If you can survive long enough to spread some damage and/or heals around, the kills rack up pretty quickly, and it probably won't take you more than two battlegrounds to get 50.
Don't forget -- if you die (and you probably will), the G.N.E.R.D.S. buff disappears, so you'll have to eat another one. If you're on the business end of overpowered DPS a lot, make sure you've got enough G.N.E.R.D.S. to cover your deaths.
Wands, like Tooth Picks, are a random reward from the treat bags innkeepers will give you once per hour, but you can't transform yourself into anything. You're likely to have a certain number of wands used on you anyway by guild pranksters or group members, but it's still common to wind up a wand or two short. Typically this has resulted in players trading wand transformations back and forth, and in past years it's been common for players to congregate in highly visible areas (e.g., around the Stormwind fountain for Alliance or on top of the Orgrimmar bank for Horde) and make offers through trade. So if you still need a few transforms (or even all of them!), just advertise.
As the holiday winds on and fewer people trick-or-treat with innkeepers, it also becomes common for players to start selling wand charges, so this is the sort of achievement you'll want to do earlier rather than later.