Here's the official list of achievements you can unlock while building your game. This list isn't intended to be exhaustive. If you've got a good idea, post it below and I'll add it to the main list here.
Sometime in the next couple of days, I'll build a little way to make it easier to display these achievements in your game's thread.
It's Just Temporary: You put in artwork in your game that you claim is temporary, until you or your friend can provide you with better art.
The Apprentice Becomes the Master: The competition ends and your temporary art becomes permanent without ever being upgraded to the final version, which was never made. (Requires: "It's Just Temporary")
Law of Live Demos: You show your WIP to somebody. Something goes horribly wrong.
Game Over: You implement the ability for players to lose the game. (Also, you just lost the game.)
Gratuitous Blood: Your explosions and deaths result in far more destruction than is necessary or even believable.
Happy Accident: You accidentally create a bug that leads to humorous results. Post the video or screenshot in the Happy Accident thread.
Time Machine: Worked for over an hour on code that you rip out later that day.
Ninja's Don't Take Off Holidays: Created an Easter Egg featuring a ninja.
Konamified: Make the Konami Code do something interesting in your game.
Head Banger: Use recursion in some form in one of your games.
8 is Enough: Use old school 8 bit graphics or sound/music in your work. Could also be a midi file on the music side.
Occam's Razor: Simplify overly complex code into something smaller and more efficient (The simplest explanation is usually the best)
You've got Class, Baby!: Comment all your classes so they are easy for others to reuse and understand. Since this one may go on forever depending on your number of classes, to earn the badge, you should hit a minimum of 25 commented classes.
That's a Lot of Text!: Comment every line of code in your project to spell things out clearly.
Vincent van Gogh: You create your own artwork for your project.
I'm a Lover, Not an Artist!: You get all, or most of your work, from online sources and do very little of your own artwork.
Special Thanks: Create a credits screen which list all those who have contributed to your project.
"Research": You spend a few hours playing a game when you should have been making one.
50 Hours: You spend at least 50 hours programming your game during the month.
100 Hours: You spend at least 100 hours programming your game during the month.
Moral Support: Leave a comment on somebody else's game thread with (constructive) feedback.
Leveled Up: You learned something that you didn't know that you can reuse in other games or other programs.
3 AM Code Fest: You spend several hours programming your game between the hours of midnight and 6 AM. Beer and pizza optional.
Prototype: You have something that could loosely be called a working game by the end of the first weekend.
Back of the Line: Don't get started until the competition is well underway.
The Thinker: Spend a hour thinking through a complicated piece of code and get it right first try.
The Tinker: Fiddle with a complicated piece of code and get it to work only after a lot of failed tweaks.
You Got Tropes: Use at least three tropes of the game chosen as the theme.
What, Writing, We're Talking About a Video Game: Create a story (however simple) and implement it into your game.
Way to Brag: Claim at least 15 achievements.
Monday: The entire weekend went by and you didn't get any programming done.
Victory: Beat your game for the first time!
The Buddy System: Spend all or part of the challenge building a game in a team.
Mission Accomplished!: The challenge ends, and you actually have a game to show for it!
Wait, We Were Supposed to Make a Game?: The challenge ends, and you don't get the Mission Accomplished! achievement.