Unlike the achievements in MMOs, I want them to be a bit more than just fun, and affect gameplay.
For example, let's say there is an achievement that can be obtained by killing 10 vampires. Let's call it "Wannabe Van Helsing" (as suggested by my muse - I think I prefer slightly humourous titles).
I'd like to give characters who complete this a tangible game benefit. For example, a small (but significant) damage bonus against vampires. Or slightly increased resistance to vampiric drain attacks.
Or for an achivement for eating every single different type of food ("I'm not fat, I'm just big-boned") which allows 10% extra nutrition from food.
I'm going to throw open the comments for suggestions for possible achivements If your suggestion is taken and used, you'll get game credit! The titles should be somewhat humourous, and referencing either common fantasty-tropes or the roguelike milieu where possible.