I think it's safe to say the assumed comic book audience has changed drastically in the last two decades.
Where the best comics were once aimed at 'the intelligent ten year old,' there is now an overwhelming trend to write toward the middle-aged man.
The problem is that comics that didn't insult kids' intelligence always brought in an older demographic, too. Maybe it wasn't cool to admit it at the time, but all that changed with Stan Lee. Soon college kids were writing in talking about Peter Parker and Jean Paul Sartre. But the most important thing to note is that Peter Parker never talked about existentialism, at least not directly. Stan Lee's characters waxed philosophical, but in the most poetic or melodramtic fashion possible. In other words, the comics were written in such a way that fans could bring weighty philosophical issues into the mix. Or they could just turn their brains off and enjoy the action and soap opera.
Some comics seem to still walk that line quite nicely, like Slott's Amazing Spider-Man, Hickman's (sadly over) FF run, or Waid's Daredevil. And yes, they can go to extremely dark places, but children's narratives always have. I'd even go so far as to argue that children's narratives are traditionally darker, because when everything is larger than life so are the monsters.
When the assumed audience was ten, you had to move the action along quickly to keep their attention. But when you're dealing with individuals who consider the newspaper light morning reading, everything changes. You can frame Avengers Disassembled through AvX as a decades long 'mega-arc' and your audience will buy into the idea with pleasure.
This could be seen as the logical continuation of what Stan Lee did when he gradually moved from one-shot stories with ongoing subplots to two and three parters. But I would argue that things have progressed to illogical extremes that undermine the immediacy of the superhero comic book narrative. Even in larger arcs, every individual issue had its own conflict that was brought to a head by issue's end (even if it was a cliffhanger).
It also reflects the way comic writers break into mainstream superheroes now. Historically, comic writers and artists learned their craft with shorter pieces that shared space in an anthology title. Up through the 80s, comic companies expected creators to know how to tell a five-page story before they could tell a twenty-two page one.
Now writers tend to either break in via work in another industry (i e animation, film, television or novels) or an independent comic book project. So it's more likely that the creator has already tackled a mega-arc or self-contained graphic novel without the traditional 20-22 page format. That makes a big difference in terms of how one approaches pacing. Animation and novels can afford longer conversations than comics.
This also impacts the visual focus of the comic IMO. In a Lee/Kirby comic, characters are always acting. If Hawkeye questions Captain America's leadership, he doesn't do it over coffee. He does it during a training session, while Cap dodges projectiles and he practices trick shots on dummies.
The conventional wisdom seems to be that slowing the narrative down allows writers to flesh characters and their motivations out more. In reality, all it does is draw attention to the inevitable logical inconsistencies involved in superhero drama. It's easier to gloss over those inconsistencies in the middle of a firefight than the Avengers breakroom, or when they happen in passing rather than being emphasized repeatedly over the course of a year.
A superhero comic is a sleight-of-hand trick, and there's a reason why those don't work in slow motion.