Recently it was announced—on "The View," of all places—that the next person found worthy to wield the power of Thor in the Marvel Universe would be a woman, after the current Thor is judged unworthy. (See here for the best write-up I've found, including an interview with writer Jason Aaron and EIC Axel Alonso.) Predictably, the comics internet went crazy, with some fans excited and supportive of the move and others very upset, many of them angry that "their Thor" could possibly be a woman.
After following the news and conversation all that day, I want to offer a few random thoughts.
1. It's a bold move on the part of Marvel Comics and I think it's a positive one. Those of us in comics fandom who want a more diverse range of characters would of course prefer that new, nonderivative female characters would be created and promoted, but we also realize the reality of the marketplace and how difficult that is. Personally, I would love to see Sif or Valkyrie put forward more—and to Marvel's credit they have tried—but it may be the case that the best way to diversify the line is substitute women and other minorities for established characters (as they did so well with Miles Morales) and hope that over time they establish themselves as beloved and independent iterations. (From what I've seen, few consider Miles to be a replacement but a fascinating character in his own right.)
Furthermore, there is a tradition of the current Thor losing his "worthiness" and the mantle being passed to another character, such as Beta Ray Bill (a horse-faced character for people like me to identify with), and we all know that Thor himself became a frog for a spell (after which an independent Frog-Thor was created, because you demanded it!). A female Thor was a natural next step in the evolution of the Thor mythos (just like I believe Sam Wilson-as-Cap is), and it allows the current Thor to go through a redemption storyline. I have no doubt the current Thor will come back, but as long as good stories are told in the meantime, I'm all for it. (And this is Jason Aaron, one of my favorite writers in the medium right now, so I have no doubt the stories will be fantastic.)
2. I think another reason this change is being met with skepticism—aside from issues with the character's gender—is that Marvel does not have an established traditon of legacy in the same way that DC does (or did before the New 52). There have been a number of Flashes, Green Lanterns (even just counting the Sector 2814 ones), Robins, and even Batmen. In particular, Wally West and Kyle Rayner served as "the" Flash and Green Lantern, respectively, for over a decade after their predecessors "went away," and a large contingent of fans embraced them, largely because their characters were more fleshed out than Barry Allen and Hal Jordan were at the time. While they were beloved, Barry and Hal's original adventures were mostly about using their powers to fight bad guys, not explore their deepest character traits, which made them relatively easy to replace with new people that, given the times in which they were introduced, were explored in more depth.
But when the Marvel heroes were created (or defrosted) in the 1960s, they were instilled from the get-go with well-defined character traits that served to make their superhero identites indistinguishable from their "real" identities. So even though there have been others to use the names, many fans don't see them as the authentic versions. With all due respect to Bucky and Rhodey, Steve Rogers is Captain America and Tony Stark is Iron Man because Steve and Tony's personalities are as much a part of the identities of Cap and Shellhead as are the shield and the armor. (Even in DC, the trinity of Superman, Batman, and Wonder Woman have become inseparably tied to Clark/Kal, Bruce, and Diana, but few others have.) And Thor takes this even further because Thor is his given name—there is no difference between the hero Thor and the "civilian" Thor (now that Don Blake has been rendered irrelevant). (Apparently the current Thor will go by a different name after he is "deposed," but even that highlights how odd this transfer of title is.)
This doesn't mean that "mantles" can't be passed on in the Marvel Universe, but merely that replacing Thor or Cap has more impact—and receives more resistance aside from issues of gender and race—than it would in DC, where multiple generations of heroes was once a grand tradition.
3. Who will be the new Thor—and who should it be? The safe bet is that it will be Angela, newly revealed to be Thor and Loki's sister. Less likely, it could be Valkyrie, another natural choice for her longlasting popularity and close ties to Asgardian lore, or Jane Foster, who recently moved to Asgard as an emissary from Midgard (Earth) and is suffering from cancer (which could be cured with the Odinforce, even though she doesn't want this). My ideal choice would be Sif, who seems to be the natural successor to the current Thor just as Sam Wilson is the natural successor to Steve Rogers (after Bucky had his shot). (As for hair color, I have it on good authority that people can change their hair color, especially if they undergo a transformation to become the Norse God of Thunder.)
4. Finally and most speculatively, I can't help but think that so many major changes at once—new Thor, new Cap, newly dead Wolverine, possible cancellation of the Fantastic Four—are increasing the odds that there is some sort of reboot coming at Marvel after this "Time Runs Out" business coming in the Avengers titles. I may be wrong, and I really hope I am, but it seems that the powers-that-be at Marvel are playing with their favorite toys one last time, taking bold chances with them, before they get all new ones. They've done radical things before—killing Cap, replacing Peter Parker, taking Dazzler seriously—but now they're doing a lot of it all at once, and frankly it worries me. Again, I hope I'm wrong, because I love the Marvel Universe and its rich history that, for the most part, the creators there respect.