After the events of the Surtur War, Surtur and Odin are trapped in Limbo, and later on it's explicitly explained that Odin stays there, forever battling Surtur, so he won't escape and threaten the Nine Worlds again. But then, during Matt Fraction's run, Odin is brought back to Asgard... Yet Surtur still remains trapped Limbo. (He got out later on, but only because of Kid Loki's machinations.) So apparently Odin wasn't needed after all to keep Surtur trapped? Why is that? And shouldn't Odin and the other Asgardians have been at least a bit concerned about the possibility of Surtur's escape, now that Odin wasn't there to keep him in check?
Baldur and company eventually took Odin's place there. So, presumably they did know someone needed to stand guard. Anyway, as now it's no longer a spoiler, Surtur got free anyway with Kid Loki's help. It was all resolved, however, by Thor killing Surtur ALSO with Kid Loki's help. It was All According to Plan. So, for the time being, the Surtur situation is resolved. He'll be back, though, as with all good archvillains.
Yeah, but Balder taking Odin's place didn't happen immediately, there was a long gap between Odin's resurrection and that. Who was keeping Surtur in check during that time? And even though the Surtur situation is resolved now, that doesn't explain why no one seemed to care about it when Odin was brought back from Limbo.
I suppose it could just be Surtur didn't have a way to get out. Both Surtur and Odin were DEAD after all, which may mean they'd need someone to resurrect them. It's just, without Odin, that undoubtedly would have eventually happened. In other words, Thor was taking a terribly stupid risk. Which is entirely fitting with his character.
In JIM, Surtur had to build up his power by eating demons that were slayed and went to limbo. As long as Odin was there to kill Surtur every day Surtur had no way to escape. As others stated, Surtur and Odin were both dead and trapped in that place without their normal level of power. It took a while for Surtur to rebuild his power once Odin was gone. By the time Balder got there Surtur was too powerful for Balder to fight, but Balder killed what demons he could to slow down Surtur's return.
What exactly qualifies someone for worthiness to wield Mjölnir? Captain America was (apparently) worthy but the Thing wasn't. Superman isn't worthy but Wonder Woman is. Beta Ray Bill, an EMT, and (if you count What Ifs) Jane Foster are worthy but plenty of others aren't. It's some damn strange criteria.
Speaking of Mjölnir's worthiness criteria, in the storyline in which Thor started trying to become worthy enough to wield Mjölnir, he was able to pick it up off the ground by about a foot or so more after each of his adventures during the storyline until was able to easily wield it, in contrast to every other issue in which anyone who is even slightly less worthy to wield Mjölnir than is required is unable to even budge the thing. It also seems that once a person has wielded Mjölnir at some point they will always be able to, even if they are no longer worthly later, like one time when Loki pulled a Body Swap on Thor, Loki in Thor's body was still able to wield Mjölnir while Thor in Loki's body wasn't, despite their minds and souls being different and the same respectively.
It has never been made clear and is up to the writers. Walter Simonson thought a willingness to kill was necessary. Mjölnir was created as a weapon of war in a warrior society so to them someone worthy would have to be willing to kill. This would explain why soldiers/warriors like Captain America and Wonder Woman can lift it, but heroes who are not warriors like the Thing and Superman cannot. This is not the sole criteria. One also has to have a high degree of noble character.
This troper believes he has found out the answer. Thor's hammer, Mjölnir, cannot be used or even lifted by any but the "worthy". History shows that this worthiness is not a common trait (even among the Avengers, only Captain America has been able to wield it), but a random paramedic, not even named, once picked it up and handed it back to him. So basically, you need to be a true hero, whether it's as a super being, Badass Normal, or even one of The Real Heroes.
Why is Thor usually among the dead heroes in the post apocalyptic universes/crossovers?
Old man Logan
Alternate universe secret wars/World War Hulk
Age of Ultron
"Hyperion takes over" reality
"Maestro takes over" reality
And these are just a few.
Probably because Thor, being a Blood Knight as well as his tactics in battle largely only amounting to Attack! Attack! Attack! without thought of retreat, is one of the heroes in the Marvel Universe most likely to rush off and face whatever threat is responsible for his deaths head on even against things he clearly doesn't have a chance against and get himself slaughtered.
On a more meta level, Thor is often considered to be the toughest of Earth's superheroes (he's basically Marvel's equivalent of Superman), so the writers are using the fact that he's dead to convey the idea that things have taken a really bad turn.
This right above. Thor is often considered overall the most powerful standard Marvel hero so killing him shows what a big bad it is. Also, he is so powerful that he can create story problems that either require him to be watered down or conviently removed. The animated film Next Avengers had Thor has a deadbeat dad who was too busy ruling Asgard to help Earth. Going by how easily Hulk trashed Ultron if Thor had been involved there would have been no film.