It may not be entirely accurate to label Avengers: Earth's Mightiest Heroes as underrated as I've never read a review of it that's been anything less than complimentary. However, I would classify it as overlooked by virtue of the fact that most people to tend to see it as just a very good comic and I think it belongs in the pantheon of truly great Avengers stories, right up there with The Kree/Skrull War, Under Siege and the seminal Kurt Busiek/George Perez run. Let's discuss why (it will be a one-sided discussion as I will be doing all the talking)...
First off, it's just visually gorgeous. I think it may be my favorite work of Scott Kolins' career, and I am a huge Scott Kolins fan, particularly of his Flash run. However, Scott's work here is not just vibrant and filled with energy as per usual, it also has a polish that you can tell came from him being able to work long and hard on a finite project. His Captain America is heroic, his Thor is larger than life, his Hulk is wonderfully grotesque and his Iron Man is a fantastic meld of retro and cutting edge. The art also has the benefit of the great Morry Hollowell, one of the very best colorists in the business, who makes the already crackling visuals pop even more.
That's just the gravy though, Joe Casey's story is the meat, and what a tender, delicious steak it is.
Casey bobs and weaves between the pivotal moments in the Avengers' earliest days to provide a stark, personal look at the men and women who lived those incredible adventures during their downtime. Away from the big fights and larger-than-life villains--though we get those too--Casey delves into what happens when you throw powerful figures with such vastly different personalities together in one place at one time. The appeal of the Avengers--and the Marvel Universe really--is that it's not a group of grinning demigods slapping one another on the back, it's a tempestuous powderkeg of clashing egos, and Casey explores that dynamic with a modern approach to classic relationships, something he does so well.
In the early issues of EMH, Iron Man gets a fair share of the focus, and Casey focuses on the stress put on a fairly ordinary (albeit rich and super-smart) guy like Tony Stark when he tries to become the lynchpin of a group that includes a tempermental monster, a neurotic giant, and a god who defies all scientific explanation. Before Civil War, we see that monstous sense of responsibility combined with more than a little arrogance that makes Tony such a fascinating if divisive figure. He believes so strongly in what the Avengers can accomplish that he becomes lost in the possibilities and appoints himself the caretaker of that potential without asking anybody's permission; in Tony Stark's mind, he's doing everybody a favor by taking on this tremendous burden because he's the only one who can handle it, never mind what they think. We all have that friend who agonizes over being the one who "has to" organize everything, but won't hear of anybody giving him a hand; Iron Man is that guy.
Casey ups the ante by showing a degree of government involvement in the Avengers' early days not present in the actual comics of the era. One of the running themes of the series is contrasting the general public's awe and excitement over the emergence of the Avengers with the government's fear over not being able to control the situation from both a PR standpoint and in terms of sheer firepower. As he has done in other similar series, Casey does a wonderful job journeying back to a time when the common man wasn't a pivotal part of Marvel history and showing where he stood.
Once the story really gets rolling, Captain America becomes the focal point, which is groovy because Scott Kolins draws a stellar Steve Rogers and Joe Casey has a good handle on his character from this period. When I say "from this period," keep in mind that the Cap of the early Silver Age was not the shining and unshakeable symbol of heroism he would become in later years, but a man out of time, uncomfortable with a world he found new and frightening and dealing with crazy survivor's guilt. The Steve Rogers who first joined the Avengers was not a well man, and while Stan Lee and others did explore this back in the day, Casey comes at it from a style of psychological storytelling that had not been refined fifty years ago. The somber and crushing experiences of Captain America trying to find his place as a man while also living up to the iconic status he had never quite prepared for is sobering and powerful. Cap's heroic journey over the course of EMH is the heart of the book; a limited series or even ongoing exploring Captain America's Silver Age adventures through a modern lens by Joe Casey and Scott Kolins is one I'd jump on in a heartbeat.
In the final few issues of EMH, the character of Hawkeye emerges, and after chronicling the stress of Iron Man and shellshock of Captain America, it's neat to see Casey and Kolins cut loose and have fun with Clint Barton. While the idea of bringing Marvel's big guns together was an appealing one, I feel like the Avengers didn't really become The Avengers until Hawkeye, Quicksilver and Scarlet Witch joined Cap's Kooky Quartet, and EMH sells that idea in convincing and sophisticated fashion. If the first few issues show what the Avengers could be through Iron Man and the middle portion shows how those hopes can be dashed by human flaw with Captain America, the home stretch demonstrates that potential realized by the heroism the original Avengers inspired in Hawkeye and how they have already transcended into legend.
I'm not doing Earth's Mightiest Heroes the justice it deserves here, but that's tough to do for such a meaty series in limited space. I would hope that if you haven't read EMH, my words have done a bit to persuade you that doing so is a worthwhile endeavor and then the morsels of Scott Kolins art sprinkled throughout this entry do the rest.