Fear Itself was one of those somewhat more contentious comic book events for Marvel since some folks loved it while others felt it was stupid but in the end not enough people bought comic books in order for it to be successful. I suppose a lot of it has to do with how difficult it is to summarize what the story is supposed to be about off hand.
Then you add in the quirks of some degree of crossover event fatigue among fans given the almost regular cycle of big events dating back at least to the time of Civil War. I mean come on - we were still coming to terms with the events of Siege and the spillover of War of Kings leading to Realm of Kings and that leading to The Thanos Imperative - Earth-616 just has way too much crap to deal with sometimes.
Synopsis: Fear Itself is a Marvel crossover comic book storyline primarily authored by Matt Fraction. The series consistent of 7 main books covering the event and a good number of tie-in titles across the Marvel universe.
The book begins with Sin, who is now the Red Skull, leading an expedition into a Thule Society fortress containing the Book of the Skull. This book leads her to the location of the Hammer of Skadi that had fallen to Earth back in World War II but at the time the original Red Skull had been unable to wield it. Now Sin felt she was fated to bear the hammer for some reason and she succeeds, thus triggering a chain of events leading to a conflict with an entity known as the Serpent, Odin's brother and supposed true ruler of Asgard.
Sin becomes Skadi, Herald of the Serpent and the activation of her hammer is part of a chain of events that summons seven more Uru hammers to Earth. As she frees the Serpent from his prison, the hammers are picked up by various heroes and villains around the world, turning them into the seven generals of the Serpent known as the Worthy. They begin to use their new powers to cause massive destruction around the Earth and thus create more fear for the Serpent to feed on in his quest for power to take on Odin. On the other hand, Odin decides to have all Asgardians withdraw from Earth as they prepare to arm themselves for the coming conflict and to raze all of Earth, thus denying the Serpent any more sources of fear to feast on. Of course Thor objects to this plan and aims to return to Earth to fight the Serpent directly.
Now on the surface, the event's main plot had a lot of interesting elements that could have been exploited given the decision to tap into Marvel Asgardian lore to to dredge up this Serpent villain as a big bad. The whole concept of the Worthy was a little weird, admittedly, and their initial plan wasn't all too clear given how the Serpent didn't directly meet with them until the end. I'm sure the supernatural powers of the Serpent were more than enough to communicate with the spiritual successors of his generals around the world, but it sort of would have made more sense if there was some greater degree of coordination here.
Plus it wasn't initially clear in the core books that the Serpent's goal was to cause more fear around the world. Some of the city-level tie-in books like the Spider-Man titles nicely depicted how there was an elevated of fear and panic being felt by people around the world. but this was not always evident in the main books.
And so it sort of felt like the generals were just breaking whatever they could with no goal or even a plan of a sort. They were just causing general chaos and were practically indestructible, even when compared to other Asgardians or other superpowered beings. And this made things feel rather lopsided since the heroes didn't seem to have much of a chance against them given their fearsome powers and just how tough they all were. If Asgard really had enemies this strong, it surprises me that they were ever able to contain them at all. That does lend some credit to why Odin was willing tot destroy Earth entirely just to prevent them from winning, but it still felt like a story pulled out of nowhere.
And given the significant global scale of the damage, I can understand some of the criticism about how it felt almost impossible for the world to get back to status quo quickly enough, and yet the nature of comic book publication cycles had to demand that thing be wrapped up quickly enough so individual titles could go back to telling their stories. Iron Man, for example, was in the middle of a weird internal conflict in his book before the Fear Itself event hit and right after he had to go back to trying to figure out who was trying to take down his new company from the inside and such. The massive worldwide damage in the event felt almost insignificant or unnecessary by the time things moved on to other things.
And the ending of things relied a lot of readers investing in a good number of the tie-in titles since the core 7 books seemed to gloss over a lot of stuff. That may make sense from the business side of things since you want to sell more comics, but that is also really annoying on the consumer side. The old events like The Infinity Gauntlet had core books that were strong enough to stand on their own with the tie-in stuff as truly supplementary material. Here you'll miss out on what Iron actually tries to do while in Asgard and other such stories that may not seem entirely part of the main plot but could have very well been still part of the main books somehow without all that embellishment.
Fear Itself was perhaps just a little too high concept to be executed well and ended up being rather clumsy given how it was spread across so many different titles. It had a confusing villain who seemed to be way overpowered for the heroes to deal with and yet they win literally through divine intervention of a sort. And so I can only give the event about 3 silly Generals of the Serpent running around the world out of a possible 5.