It is one of the great creative ironies in superhero comics that those characters with the most distinctive and colorful individual milieus are often less popular than characters with more generalized, which is not to say generic contexts. Superman has Metropolis and Batman has Gotham, but these are both just cross-eyed approximations of New York City; Spider-Man lives in the real New York City. These are all colorful backdrops but they're still situated securely enough in recognizable reality (or its spandex approximation) that the characters themselves can go in many directions. I would even go so far as to say that these characters are more modular as a direct consequence of their open-ended milieus, and this modularity gives them room enough that they can reasonably expand to fill an almost infinite variety of storytelling spaces. All of these characters have in the past anchored long-running team-up anthology books, and their respective popularity has dictated their status as utility players, able to be plugged into almost any different story type or genre simply by virtue of their ubiquity.
The concept of modularity, while vital to the longevity of characters such as Spider-Man, Batman or Superman, is a double-edged sword when applied to characters with less familiar milieus. Namor and Aquaman are perfect examples: both of them have extremely well-developed (and, it must be said, extremely similar) fantasy milieus - i.e., the underwater world of Atlantis and the mysteries of the deep. But because of that, it's far harder to extract these characters from their milieus, and as a result their stories can become quickly repetitive. Superman is based in Metropolis but the nature of the character is such that he can have any kind of adventure - from a gritty crime drama in Suicide Slum to a slugfest with Darkseid at the far end of the universe. Similarly, although in theory Batman might not initially appear quite as flexible, Batman can and does "work" in multiple contexts and even multiple genres. He has a well defined "home base" in Gotham City and the criminal milieu, but he can interact with the JLA or travel in time to fight Jonah Hex because his character has a strong enough "thru-line" to maintain cohesion. Spider-Man, similarly, works in almost any context because his "thru-line" is elementary: he's still the kid from Queens who is perpetually punching above his weight class, and his awareness of his misfit status keeps him recognizable despite having fought Thanos and the Beyonder for the fate of the universe at various points. You can't quite do so many things with Namor or Aquaman - they're not bad characters, but the fact that they are deeply tied to such a well-defined fantasy setting limits their applicability. There is some truth to all the old joke about Aquaman being useless: it's not like he can't do many things, but that his character is such that it's there are many types of stories which simply won't work, or would be superfluous. You could write an Aquaman story about drug dealers in Hell's Kitchen, but why? There's nothing about the character that complements an urban crime story, or a political story, or an Agatha Christie mystery. The difference is that you could write a Batman story about evil whalers or deep-sea polluters without it being particularly unusual. (And, to be sure, Batman has fought more than his share of nautical foes.)
Of course, the reason those characters have become so flexible is a product of their popularity: it's almost certain that Bob Kane and Bill Finger did not initially envision a Batman who wore zebra colored costumes, interacted with omnipotent comic-relief imps, fought immortal Bedouin warlords in the desert with cutlasses under the moonlight, or faced down the god of evil with a gun that fired time-traveling bullets - and yet, because the character was popular he was shoehorned into all these different types of stories, and because many of the creators who wrote these stories were very talented, they worked. It's accepted by now that Batman - as well as Superman, Spider-Man, and probably a handful of other evergreen characters - are resourceful utility players who can survive and thrive in a dizzying variety of storytelling environments. They have their core strengths, sure - Batman always reverts to urban crime, Spider-Man always returns to hard-luck soap opera - but they are so enduringly popular not just because their core concepts are so strong but because their core attributes are flexible enough to withstand an almost absurd amount of abuse.
Some characters, regardless of how strong their core concept may be, just aren't very flexible, and this lack of flexibility (real or perceived) impacts the different kinds of stories that can be told. And if the marketplace is not friendly to whichever type of story your character might inhabit, your character will be relegated to the back bench. The conventional wisdom goes something along the lines that Namor and Aquaman aren't interesting because only a small sliver of the audience will ever care about Atlantis. Only a sliver of the audience cares about Dr. Strange's magic worlds or the Silver Surfer's infinite galaxies. The logic goes that these characters are somehow limited despite the ostensibly open-ended nature of their milieus - because, even though these characters' contexts offer an infinite number of variations based on their simple premises, they're still stuck in set genres. Dr. Strange is a monstrously strong high concept that should allow for an almost infinite variation on the same essential theme. But, the sentiment follows, if you don't like high fantasy, you're not going to like Dr. Strange regardless of how fantastically trippy his adventures may be. Similarly, if you don't like sci-fi space opera, you're going to be left cold by all the different variations on space wars and cosmic deities the Surfer encounters. And, it goes without saying, even though the surface of the world is 3/4 water, if you don't find the sea inherently interesting you're not going to want to read Aquaman. If you don't like those kinds of stories, you're not going to like even the best variations on the same theme. This explains why so many of these characters' periodic revamps begin with an attempt to remove whichever character from their core milieu - Dr. Strange isn't "working" as traditional fantasy, or at least the audience isn't responding to him as such, so lets pare down the fantasy elements and turn his milieu into one that more closely resembles another, more popular genre. Or let's do a Silver Surfer series that isn't about flying through space and fighting monsters, but sad people sitting around hospital rooms. Etc etc, ad infinitum.
We are left with the perception that some characters are just fated to be less popular than others by virtue of being stuck in a secondary or tertiary genre - there is a ceiling to how popular they can become in their traditional form because the audience for their customary genre is limited. However this is not, strictly speaking, true, and the reason why this isn't always true is that this supposed rule is broken all the time. The rule only works when companies are trying to explain why something doesn't sell, but when something does sell and sell consistently, what might have been an exception to the rule (such as, say, the occasional flurry of interest in tertiary characters like Hawkman or Ghost Rider), becomes the new rule. If you were to go back in time seven or eight years (to, roughly, right before Reborn) in an attempt to convince people that Green Lantern would be as popular as the the X-Men by the end of the decade, you'd have been roundly and rightly mocked. Green Lantern is high fantasy masquerading with sci-fi trappings - all things that have had particularly spotty commercial track records these last couple decades. And yet, for a number of reasons, the resurrected Green Lantern franchise has been DC's most dependable and dominant moneymaker for a few years. Suddenly, people liked cosmic, sci-fi flavored spandex in a way they hadn't at least since the days of the Infinity Gauntlet, and this success was also reflected - albeit to a lesser degree - in the surprisingly long runs of Marvel's various Annihilation branded series and spin-offs.
On paper, Green Lantern is a "limited" character, or at least a far more limited character than Superman or Spider-Man. While in theory his appeal can be explained by recourse to the wish-fulfillment aspects of the character, in truth there is precious little in the way of wish-fulfillment in a magic ring that entitles the bearer to receive . . . unlimited access to the office politics of surly blue dwarves. But with the right coat of paint Green Lantern's milieu became interesting to a new audience. The archetype of this model is Swamp Thing, who was seen in the early 80s as a definitively limited character, tied to an uninteresting backstory and generic milieu. And then, of course, the right writer came along with the right approach and that changed overnight; what had been regarded as limitations were inverted and what had been a very limited canvas was transformed into something far richer than anyone could have reasonably predicted.
Thor was a problematic property for a very long time. The colorful fantasy setting that gave Thor his distinctive feel was perceived as a high barrier to entry: if you didn't care about a fantasy world of bland demigods living apart from Earth and substantially separated from the Marvel Universe, you weren't likely to care about Thor. When the Punisher, Wolverine and Ghost Rider were the hottest characters in comics (ie, the early 90s), it was hard to convince people that old hands like Thor, Iron Man and Captain America weren't also old hat. It didn't help that Simonson's popular run was followed by DeFalco and Frenz's solidly retro-flavored tenure. Don't get me wrong, I loved their Thor, and it certainly holds up better than most of the proto-Image artists' runs on the "cooler" books, but it was very resolutely square, and a large percentage of Marvel (and DC's) line at the time was perceived as similarly square for many years after. For proof, look at the fact that they actually signed off on loaning these core characters to the same Image creators who had initially left just a few years prior, a decision made in the hopes of spreading some heat onto characters who didn't wear X'es on their belt buckles. It became conventional wisdom that these characters were profoundly broken, and needed shock treatment in order to bring them into the 20th century. But ultimately, Heroes Reborn was a shot-lived stunt, and the only thing that was able to build interest in these characters over the long term was a sustained and radical reinvention.
"Reinvention" here does not necessarily mean wholesale change - but in order to invigorate very old and seemingly tired characters, they needed to discover new ways of telling old stories. Geoff Johns' Green Lantern isn't telling fundamentally different kinds of Green Lantern stories, but it's telling them in a way that seems novel and exciting to a plurality of readers. Likewise, there is very little new in JMS' Thor, but the way the stories were told was quite novel in terms of reader expectations regarding how Thor should read. Neither Green Lantern nor Thor saw their milieus significantly altered in their recent revamps, but the creators behind the books spruced up once-tired archetypes through a series of novel creative decisions. What had once been limited, ostensibly stifling canvases opened up and became reacher and more resonant with a larger number of readers. Had the characters themselves changed? Is Hal Jordan a more flexible, modular character now that he's leading a hugely successful franchise? Perhaps his success allows him a modicum more flexibility, but he'll still never be the utility player that Batman is; similarly, as popular as Thor has been recently, it's still hard to envision him anchoring any type of team-up book in the near future. But they're popular characters nonetheless because someone sat down and put some thought into exactly how their milieus could be made more interesting to a larger group of readers. Most importantly, however, is the question of whether there are now more types of Thor stories that people can tell.