Unpopular opinion time: I don't think MCU!Mjölnir functions via the whole worthiness thing. My main reason for thinking this comes from how Odin doesn't really apply that enchantment until Thor's banishment. The runes on it don't mention worthiness at all, they just indicate the wielder 'commands the lightning and the storm'. So I guess that leads to another question: once Thor was 'worthy' again in the first movie, was the enchantment over? I think this is the case. And what that means is that unless you know how to wield Mjölnir, you can't. It's not that Loki or anyone else isn't worthy, they just don't know how to make it move. For Loki and other Jotun/Aesir/etc., this could just be about different aspects and specializations of magic. It's not a kind of magic he's familiar/comfortable with, just like Thor pretty much can't deal with a glamour to save his life, so until he figures out how to--which might not be possible--he won't be able to lift it. I saw a really good post talking about how Mjölnir won't move for others not because it's heavy, but because it's stubborn. I like this as a concept; i.e., the density and enchantments on it are what makes it powerful, but magic also prevents it from being a gravitational problem, and magic is what allows Thor and Odin and Algrim (when he bats it aside in The Dark World) to move it. They're the only ones who ever do, and I think this makes a case not for worthiness--Odin is a pretty big jerk, IMO, if not the most horriblest person ever, and Algrim killed Frigga for the high crime of protecting Jane, not exactly a sign of worth, and Thor smacking Tony around is not exactly worth incarnate--but simply for knowledge of how to work with it. Worthiness is also subjective and what Odin thinks is worthy may not track with anyone else in the universe. So until we've got something more concrete in the films to countermand it, that's what I'm rolling with.