Nick Fury whispered *something*, a secret in Thor's ear. As soon as he heard it, he immediately became unworthy to wield Mjolnir. Jason Aaron, the writer of Original Sin and the current writer of Thor, swears that he knows what that secret is and why if affected Thor as it did. He's promised that it will be brought up in the current series soon but the focus right now is on Jane Foster as Thor.

I wasn't aware of that issue of What If? till I was talking to some other friends about the reveal of Jane being the new Thor. I'm unfamiliar with both the writer and artist of that issue but judging by their comments (and yours), it sounds like a fun issue. It certainly seems likely to have been a huge influence of Aaron's current work. Jane's current Thor costume is a slight variation of her look in that panel (spun gold hair and all). But IMHO, Aaron is doing a terrific job in crafting Jane's character, motives, and thoughts as the new Thor.

Dude, It was in the back of my mind when it came out but you just put it into words for me. When Thor became Unworthy, he must have known it at the time. But what could it be? He has killed before and even raped before. 

Me too. His reaction wasn't like he was wondering what it was. He must know what he did.

I think it would have to be stronger than negligence. It's about losing honor, I think.

 
Okay you're right.

It must be a past action that he didn't know had dire consequences until it was whispered to him. And his subsequent feeling of guilt made him unworthy. I am still going to go with a race of people getting killed by his inadvertent action.

There's a couple of ways of viewing this.

1) What does it exactly mean to be 'worthy'? Is it solely a condition of the hammer's spell or does it have do with the wielder's perception of themselves? For example, the times that Thor has been mind-controlled in some way and directed to attack the Avengers (Moondragon comes to mind), he was still able to wield Mjolnir in doing so but clearly such an attack isn't 'worthy'. Unless in his mind-controlled state, he believes that new goal is worthy and thus he can still wield the hammer.

2) Aaron has been laying subtle hints that Mjolnir may in fact be more sentient than first thought. That the original spells and magic infused upon it have evolved into something new. In fact, Odin can no longer wield nor command Mjolnir. He tried, first after Thor dropped it and Odin tried to recover it, then when he tried to retrieve the hammer from Jane after she picked it up.

So if Mjolnir is sentient to some degree, then how did the hammer not know of the secret Fury whispered? It goes back to some hidden aspect of a previous action (or inaction) that Thor committed.

It's frustrating to a degree but at least it hasn't been brought up too much by any one character lately.

(Plus, didn't mean to hijack this thread. I've always appreciated the What If? series but haven't read all that much of the original run. I still have the Miller issue of What If Elektra Lived? and I remember liking the What If Thor met Conan? issue [where Conan actually became the wielder of Mjolnir at the end if I recall correctly] Yikes!)