I was thinking back on my little DC vs. Marvel post from Tuesday, and the Fortress Keeper's statement that Superman should be able to wield Thor's hammer, without any special help from Odin. So that got me thinking about what exactly makes someone "worthy" of lifting the hammer? As Wonder Woman pointed out in DC vs. Marvel #2, the word "worthy" is awfully subjective.
I think it may be a matter of Norse culture, or comic book interpretation of Norse culture. "Norseman" equals "Vikings" to most people, assuming they even know what a Norseman is. In my experience, when people think of Vikings, they think about violence and pillaging and being a warrior. And The Mighty Thor takes pride in his prowess as a warrior, he enjoys a challenging battle, it's generally regarded as a good thing (as long as friends and loved ones aren't being harmed, then he just gets really angry). So, maybe that's the key?
Looking at the people who can wield Mjolnir, you've got Beta Ray Bill, who was designed to be the ultimate warrior, keeping his people safe from Surtur's forces as they scrambled for a new home. He endured considerable pain, physical and psychological to reach that point. You've got Captain America, underwent dangerous experimental trials to be a soldier who could serve his country in time of war. His goal is to fight evil wherever it is, to preserve freedom. There's Wonder Woman, who at least in one origin had to pass several trials to earn the right to leave Themyscira, and is a great warrior as well.
Don Blake was just an identity Odin concocted to teach Thor a lesson, so he doesn't count. Eric Masterson was always sharing a body with Thor, so I argue it was Thor enabling him to use the hammer, even with Thor's spirit had supposedly been banished. By the time the separated, Masterson had proven himself as a great enough warrior to be able, like Beta Ray Bill, to wield a mystical hammer of his own. Too bad he wasn't a coward, I think Marvel could have done without Thunderstrike. There was Dargo from the 26th century, but his tenure was marked by Mjolnir trying to get the hell away from him, so he doesn't count either.
So it seems like a common thread is that people who can wield Mjolnir are great warriors that underwent trials to gain their abilities/gifts. Could that be Superman's problem? Even though he died in battle protecting others, so did Skruge and I don't see him chucking Mjolnir around. It doesn't seem like Kal-El has ever had to fight for his abilities, or go through trials and tests to gain the power he has, so maybe that's held against him. Of course, Thor didn't have to either, being born with considerable power and then getting Mjolnir as a gift, but I'd say his father's prerogative wins out there, and he's certainly fought enough epic battles to have earned it since then.
Keep in mind, I'm not arguing Superman shouldn't be able to wield the hammer (though I enjoyed watching him fail to pick it up, and I really wish Captain America had casually walked by, picked it up, and tossed it to Thor, just for the look on the Kryptonian's face), I'm just providing a possible reason why it could be argued he can't. Of course, I'm not up on Superman history, so I've probably missed something in his history, though I don't consider getting over his own subconscious desire to be normal a worthy trial.