If there's one moment that I think defines Bendis' New Avengers for better or worse, it's the very first scene of the first issue, where a shadowy figure offers Max Dillion ( a.k.a. the villain Electro ) a job, and tells him that it's up to him how he wants to dress for it; costume or no costume. Dillon puts on his lightning mask, gives us an evil grin, and says " costume ".
This, in microcosm, set the stage for the way Marvel would be doing the next few years of stories, and the way they continue to write. Keep in mind that New Avengers came at a period when Marvel, under the direction of controversial ( and at that point, recently absent ) president Bill Jemas, had rebelled against their roots. Their biggest hit was the Ultimate Universe, which reinvented the characters from scratch in a more realistic and subdued context, dramatically simplifying or removing the costumes ( except Spidey ), largely avoiding stories about magic and gods and grounding everything in either the Super-Soldier Formula or the mutant gene. Many of their franchises had been made over to reject superhero genre tropes; the Grant Morrison X-Men most notoriously, having ditched costumes altogether and explicitly denying their superhero identities. But we also had the Bruce Jones Hulk, who simply wandered the countryside in a conspiracy theory variant of the old TV show; Captain America in the Marvel Knights imprint, spending more of his time fighting Islamic fundamentalists than costumed criminals; even Bendis' Daredevil, which used quite a bit of the old continuity, made the distinction between Matt Murdoch as superhero and Matt Murdoch as pulp vigilante. Marvel was not really publishing non-superhero material, but they weren't comfortable with their old characters.
Whatever your opinion is on this rebellion against Marvel history, it was not obvious in the Avengers of the time, and one has to admit that they suffered for it. The Avengers remained the traditional superhero team living in a posh mansion and fighting costumed criminals. The team had writers who kept them in that status quo, most notably Kurt Busiek, but also Geoff Johns and Chuck Austen. In the meantime, Marvel was also publishing the Ultimates, which gave us a strikingly warped version of the characters. And the market seemed to prefer the latter; Tony Stark as a repentant do-gooder in form-fitting metal tights wasn't as dynamic as Tony Stark as a decadent neo-liberal in a humanoid mech. Regardless of individual tastes, Ultimate-style comics were where the discussion was; to try and ignore it was folly.
So thus we get to what Bendis did, which wasn't to deny the superhero genre's trappings-- but not to embrace them, either. The way Bendis writes the Marvel characters, being a superhero or a supervillain is a statement; you wear a costume as a conscious choice about what identity you're projecting. Electro is a villain who does not need a costume; when he's powered up, he's spewing electric sparks everywhere, so wearing a green leotard with a yellow lightning mask is redundant as well as ridiculous. By putting on the costume, he's putting himself out there-- in that issue, he arrives at the metahuman prison called the Raft, breaks all the convicts out, and stands before them in his costume, dramatically telling them that they owe him. The costume isn't just a fact of the business for Max Dillon, it's an emblem of why he's in this business-- to look, feel, and act powerful. Even though they use their power to help instead of hurt, superheroes have the same motivation.
And this goes not just for costumes, but superhero genre trappings in general. Since New Avengers, Marvel's publishing landscape has swung back towards comics' historical excesses. Shared universe crossover has increased, with New Avengers and Mighty/Dark Avengers being the center and every other comic following their lead. Stories aren't written as much like done-in-one movies in trade paperback form, with long, subplot-filled sagas like the Death of Captain America back in vogue. And history is once more a motivator for contemporary stories, from Bucky coming back from the dead and eventually taking on his mentor's patriotic identity, to Spidey's dead ex Gwen Stacy having been impregnated with Norman Osborn's gobliny children ( the former being a good example, the latter being one of significantly lesser quality ). But they're doing this largely with the awareness that they're working with a very specific genre, and trying to expand or subvert it. I've talked about Bendis' Avengers being very conscious of their image, but even solo books, like Matt Fraction's sci-fi-driven Iron Man or Ed Brubaker's political thriller-inspired Captain America, have made the superhero heritage of their books more deliberate in the new era.
One could say that this mixture of old and new leads to greater work; one could also say that it's a faulty compromise that gets away from what makes superheroes work. But it's the way Marvel's been for years, and Bendis' New Avengers has led the charge.