As for the hammer itself, I'm going to go more with what tradition says and say that its not a huge two handed marvel comics kind of hammer but rather a blacksmith style hammer that's meant to be used one handed.
Actually, the traditional Mjolnir (which means "crusher" in old Norse) was a huge, 2 handed hammer, with a shortened handle.
Loki cut off Sifs hair, and to avert Thor's rage, he promised to go to the great dwarf smith's and return with new golden hair for Sif, aswell as gifts for Odin, Freyr and Thor himself.
Upon reaching the hall, Loki bet the dwarf that he couldn't craft these objects of unsurpassed perfection. The stake for this bet was Loki's head.
The Dwarf began forging the gifts (which requires him to never cease working the bellows)and Loki decided to cheat to insure his victory.
Loki, in disguise as a fly, comes and bites Brokkr on the arm but Brokkr continues to blow. Then Eitri takes out Gullinbursti which is Freyr's boar with shining golden bristles.
Then Eitri puts some gold in the furnace. Loki in the fly guise comes again and bites Brokkr's neck twice as hard. But as before nothing happens and Eitri takes out Draupnir, Odin's armring, with the power to create 9 duplicates of itself every ninth night.
Eitri then puts iron in the forge to create a mighty warhammer and tells Brokkr to never stop blowing. Loki comes again and bites Brokkr on the eyelid much harder than before and the blood makes him stop blowing for a short while. When Eitri comes and takes out Mjöllnir, the handle is a bit short (making it one handed).
Loki brings the gifts back to Asgard assured in his victory, as the shortened handle of the warhammer would make it impossible to wield. However, upon presenting the hammer to mighty Thor, he declares it perfect, as the shortened handle will allow him to wield it one handed (with the aid of his girdle and gauntlets of strength) and thus swing it more swiftly.
The gods declare the Dwarves the victor and Lokis head forfeit, but at the last moment, Odin decrees that while the wager was for Lokis head, it said nothing of his neck. And therefore if they caused any harm to his neck, the Aesir would be forced to kill them.
Thus, cheated out of their reward the Dwarves settled for sewing Lokis mouth shut to teach him a lesson.
So, really the traditional Myth of Mjolnir (and most depictions of it) are of a massive maul with a short handle.
Now I do agree that it should return to the thrower, and that it should have a high weapon rating. but I also think it should require at least supernatural strength to wield (maybe Mythic to throw), probably should satisfy the catch on all giants and trolls (Thor was predominantly a giant killer) and maybe give it the power to call lightning (its the ultimate symbol of thors power).
That last brings me to the most important question. How is anyone going to get it away from Thor?