What a great holiday! You get to dress up as whatever you want! That is fun. Sometimes you go to parties and everyone gets wasted.
11.) The Person That Goes As What Everyone Else Is:

Clever guys!
Last year, the costume was Borat. This year? The Joker and Sarah Palin. Don't be the person that fails to realize that if you think you're breaking ground, you're probably not.
10.) The Person That Uses Halloween As A Fantasy:

The jocks won't mess with us tonight. NOT WHEN OUR POWERS COMBINE.
This is pretty general, so we'll explain. EVERYONE uses Halloween as a fantasy. You can dress up as whatever you want. Can we just dial it down a few degrees on the whole "acting the part" thing that goes around? You're Spiderman, everyone really appreciates that you sprayed silly string in the punch bowl.
9.) The Person That Failed To Think This Through:

I don't have to use the bathroom. I just went. Only, not in the bathroom.
You put 80 man hours in to create the most realistic Metal Gear Rex costume anyone has ever seen! But wait! You will be unable to drink, go to the bathroom, see people, move, or breathe. OOPS! Was it worth it?
8.) The Person That Is Also A Handyman:

It only took 7 months!
You've seen this person. They've managed to construct their costume out of lumber, copper, steel, and an old car. It has moving parts. Can't this guy just go to a "Spirit" superstore like everyone else?
7.) The Person That Just Adds "Sexy" In Front Of A Noun:

Ironic that eating enough of these means you can't wear this costume.
Possibly the most visible costume on Halloween, this involves a little knowledge of the English language. Noun: person, thing, place. Put the word "sexy/slutty" in front of one of these. There's your costume! Example for a person: "Sexy Sarah Palin." Example for thing: "Sexy Coffee Pot"; Example for place: "Sexy Cincinnati."
6.) The Person That Shouldn't Add "Sexy" In Front Of A Noun:

Hey lady, you forgot to not wear that.
This person takes the aforementioned rule, somehow ignores their obvious love for dipping french fries in whipped cream, and makes a revealing costume. Unfortunately for them, "Sexy Rosie O'Donnell" just doesn't work.
5.) The Person Who Is So Clever:

See? I'm Youtube. It's topical.
"Check this out, my costume is a double entendre! I'm a minor/miner! Get it? A coal-miner who is also under 18! Or sometimes they're a figure of speech. Like the guy who dresses up as a bush with two birds in it, and holds a bird in one hand. No explanation needed. But sometimes, this costume can just be something from a movie or TV show that's super obscure. Like a monster of the week from the X-Files, or the "Soup Nazi" from Seinfeld.
4.) The Person That Is Too Proud Of How Much Their Costume Cost:

Guy on left: spent too much. Guy on right: didn't spend enough.
There is no middle ground. If you were able to make your costume for as little as $5, awesome! If you're a big spender, no one cares either.
3.) The Person That Goes As Him/Herself:

Paris, you were supposed to wear a costume...
This is the person that either has no idea that it's Halloween, or thinks it's hilarious that they've tossed on their favorite sweater and proclaimed that they've created the ultimate irony: that by not wearing a costume, they actually ARE wearing a costume, and that costume is THEIR OWN SELF. Wait, that's just retarded. They're just "too cool" to dress up.
2.) The Person That Sees Tragedy As Opportunity:

Look at this lineup of grade-A assholes.
Sadly, there's a human tragedy often enough for this sadistic go-getter to be supplied with offensive Halloween costume ammunition every year. A few years ago, it was Steve Irwin. Last year, it was being a Virginia Tech student. People have even been seen around town, clad in September 11th costumes. Be sure to kick this douchebag out of your party.
1.) The Person That's Pathetically Trying To Get Laid:

They even managed to find a model that LOOKS like a douche.
Usually it's a guy. His costume has something that suggests a girl perform certain acts on him. Cool, man. They act like it's a joke, but we all know it isn't. Not by a long shot.