I'm pretty sure I've designed more achievements in games than anyone else on the planet. I don't make this claim lightly. I've been making badges at Kongregate for 10 years now, with nearly 3,000 achievements total. I wrote a popular Gamasutra article about how to design them, and later spoke at GDC about achievement design. I've gotten this down to a science. Play a game (until you know it inside and out), identify potential tasks, filter out the bad ones (see above Gamasutra article), expand on the best ones, carefully set the thresholds (not too easy, not too long/frustrating), decide what difficulty it should be, ensure it highlights the best the game has to offer, make sure all the technical API submissions are working properly, find a relevant image, crop/resize it, BOOM, done! ...except, wait, no, it needs a name. Dammit. That's always the hardest part. Whenever I speak with developers during the achievement-making process, I always give them a chance to suggest names if they want, and about 90% of the time their response is, "