rr notes that
Loki tricked the
dwarves out of taking his head. According to the versions of the
myth I've read, he did this by pointing out that the bet entitled them to his head, but not his
neck. Presumably they also felt it only allowed them to get his
whole head, otherwise they could have just bisected his
brain or something. There's also the fact that Loki was
Odin's
blood brother, and so taking his head would involve a
blood-debt against the
All-Father. I don't think this is mentioned in the recorded version of the story, but it certainly would have been on
my mind.
Anyway,
shades of Shylock!
Mjollnir also appears in the hands of
Marvel Comics'
superhero version of
Thor. There it has the additional property of causing lightning to strike a target of Thor's choosing when the handle is rapped against the ground (interesting, because the
mythical hammer had a short handle, such that it probably wouldn't portrude beyond the bottom of Thor's closed
fist). Also, no one but Thor is supposed to be able to lift it, although this has been violated a few times (among them, by
Captain America).