I've also seen achievements used for humor and story purposes; they can provide that bit of levity that helps ameliorate an excessively dark moment, or turn a nasty trap into a chuckle, or provide a fourth-wall-breaking moment.
For an example of that last case, see the Portal 2 achievement "This Is The Part Where He Kills You".
- http://magicka.gamepedia.com/Let_off_some_steam (Notice the logo)
- IMMA FIRIN' MAH LAZER!!!!: Successfully cross two beams into a more powerful one.
- It's over nine thousand!!!!: Deal over 9000 damage to one enemy.
Well-made, well-placed achievements certainly do have a place, even in casual gaming.
What kinds of edge cases came up in testing that would be fun for the player? What kind of super secrets can you rally your playerbase to find/hunt down? What kind of silly little tasks can you come up with?
For example, in WoW it got me to go explore all of the other faction's territories, which I didn't even know was possible. It also got me to run the old dungeons that no one did anymore.
When designed right, they can be really good.
Further development ideas for retrophies inclued building on this community aspect. Although now that I've discovered retroachievements.org I'm not sure it's worth the duplicated effort :P