Marvel Studios has rewarded the loyalty of its fans in a lot of ways (chief among them the simple fact that they produce consistent, quality content), but here I want to specifically highlight the way they reward loyalty to fans of the comicbooks, to people who enjoy accuracy to the source material. I had the thought recently that Marvel Studios often plays the long game; they start with some elements of comicbook lore being more grounded or restrained, then when the characters or basic concepts have been successfully introduced, in later movies they fold back in additional elements that are more faithful to the source material. Sometimes they even do it within the span of a single movie (such as the Red Skull appearing to have a normal face at first, and then later revealing it was only a mask, hiding his comic-accurate look), but it's more common for them to wait and let the viewers get used to the basic concept before making it more fantastical.
Now, let me clarify for the people who aren't comics fans; I'm not stating these elements as being intrinsically better purely because they follow the comics. There can be situations where a movie is better off deviating from the source material, but from the standpoint of a comic fan, it always makes me happy when they try to bring things closer to the source. Personally, I don't understand why people bother to adapt things if they're not gonna try to honor the source material, but that's a whole other discussion…
That being said, here are seven different times Marvel Studios has rewarded comic fans' loyalty in that way.
In Iron Man, SHIELD was only represented by a few guys in suits. When SHIELD came back in Thor, it was more thoroughly staffed and expansive, but still just guys in suits or the standard black-clad special-ops types. It was a far cry from the high-tech operation of the comics, with their form-fitting blue uniforms, sci-fi guns, and Helicarrier. Finally, in the Avengers, Joss Whedon gave us something a little more like the SHIELD of the comics, complete with tight blue uniforms, a Helicarrier, and one rather large sci-fi gun for Agent Coulson.
In Iron Man 2, the closest thing we got to an acknowledgement that Black Widow was Russian was admitting that her real name was Natasha Romanoff, rather than Natalie Rushman. There was no mention of either her Russian heritage or her villainous past. Then, once again in the Avengers, her very first scene had her speaking Russian, and a later character-defining scene dug into her sordid, violent history. The Black Widow of Iron Man 2 was a cipher, just the form and function of Black Widow without the heart or soul. In The Avengers, she became Natasha Romanoff for real.
[Spoiler warning for the few people who haven't seen Iron Man 3 yet but still don't want to be spoiled on it; and also for the one-shot, All Hail the King.] In Iron Man 3, all the trailers and prerelease materials perpetuated the misdirection that Ben Kingsley was playing the Mandarin. In the middle of the movie, however, we find out that Kingsley's character (Trevor Slattery) is just an actor who's pretending to be a larger-than-life radical as a front for the machinations of AIM's Aldrich Killian. In essence, they basically just ripped off Ra's Al Ghul from Batman Begins.
Then, in the One-Shot short feature, "All Hail the King," we go behind the scenes of a documentary-in-progress about Trevor Slattery, giving us a look at his prison life (in Seagate Prison, where Luke Cage and possibly Scott Lang are also prisoners), and a little background on his acting career. It's all very light and quite funny at times, until the very end when the man making the documentary reveals himself to be a highly-skilled operative of the real Mandarin, who's rather upset at Slattery for appropriating his name and persona. Of course, without Iron Man 4 happening anytime soon, it's unlikely that we'll be seeing that thread followed up on in the foreseeable future, but the simple fact that there's now canonical verification that a real Mandarin exists is a huge gift to fans of comic accuracy.
In Captain America: The First Avenger, Cap was supported by a diverse group of soldiers, most of whom were borrowed from the comicbook, "Sgt. Fury and His Howling Commandos," but in that movie, the group wasn't given any official name. In Captain America: Super Soldier (the video game spin-off of the movie), the group was referred to as "The Invaders," a name taken from the team of World War II superheroes in the comics (and one that's a lot more restrained than "Howling Commandos"). However, in Captain America: The Winter Soldier, Steve visits a museum exhibit honoring the efforts of Cap and his allies, and there, they are referred to by name as The Howling Commandos.
Now, it may not seem like the mere reference of a group title is worth making an entire item on this list, but I think it does demonstrate how Marvel Studios cares about upholding the source material. As the legend goes, Stan Lee chose the name "Sgt. Fury and His Howling Commandos" on a sort of dare, specifically because of how ridiculous it sounded. So in adapting the comics to a movie that you want people who aren't comic fans to take seriously, it would seem like a name specifically intended to be ridiculous should be the first to go. But in The Winter Soldier, they were referred to as the Howling Commandos, for no other reason than that's what they were called in the comics. It's as if the people who made that decision were saying, "we don't care if this is ridiculous, we're gonna do it because it's true to the source material, and hey, guess what, notice how it doesn't have any negative effect on the movie." It was an attitude that was played out much more overtly in various ways throughout Guardians of the Galaxy, and I'm confident such acceptance of the inoffensively ridiculous parts of comics will continue to be seen in further Marvel Studios movies.
In the comics, Zola was a Nazi scientist who transferred his consciousness into a robot body. The robot had no human-shaped head; instead, an image of Zola's face was displayed on a screen in the robot's torso. Of course, something like that is a little much for a live-action movie, so in Zola's first appearance, the closest we got was an in-joke reference of Zola's face behind a magnifying glass. But after that, Zola's comic-accurate form was paid off in two different ways. Most of the people who would bother to read this post will probably have seen Captain America: The Winter Soldier, in which Zola's consciousness is preserved in a computer system, with an image of his face displayed on the central monitor. It's certainly not as mobile as a robot body, but still pretty similar in a lot of ways. But even before that, in Captain America: Super Soldier (the video game which spun off of The First Avenger to tell an additional story of Cap's adventures during World War II), we do get to see the full comic-style Zola's mind-in-robot-body with his face on a screen in the robot's torso (the video game didn't keep up with the continuity of the movies in a few ways, so it's definitely not MCU canon, but it is pretty close).
In the comics, Edwin Jarvis was the butler in Tony Stark's mansion which became the home of the Avengers, and so he became the butler of the Avengers for many years. Of course, that's too similar to Alfred, so they would never do that in the movies, right? (Fun fact, Jarvis was introduced in the comics not long after Alfred was apparently killed.) Instead of an old English butler, Jarvis was reduced to being merely a voice in Tony Stark's ear throughout the series of Iron Man movies. However, the actual human character of Edwin Jarvis has been announced as a regular character in the upcoming Agent Carter TV series. I doubt seeing some stuffy old English butler was high on many people's comic-adaptation wishlist, but it still is kinda fun just because it's another element pulled from the comics, now brought to life onscreen (I still think it would be pretty funny to see what kind of hi-jinks someone like Joss Whedon could come up with if the Avengers themselves had a butler, but this is good enough).
In some ways, the series as a whole has exemplified this concept of Marvel starting with something very basic and then rewarding the loyalty of comic fans who stuck with it. At the beginning, it was a small crew (comprised of Coulson, two field agents, two brainy types and a hacker) taking on the usual villain-of-the-week conflicts independent of the larger SHIELD infrastructure we finally got to see in The Avengers. But with the events of The Winter Soldier causing massive upheaval to that infrastructure and the overall status quo of what it means to be a SHIELD agent, the structure of the show has effectively unmade and rebuilt itself in an image much more like the SHIELD of the comics.
The scope is still much smaller than a fully-staffed Helicarrier, but we are now seeing something much more like the original Lee/Kirby or Jim Steranko stories of SHIELD vs. Hydra and spy vs. spy (with plenty of larger-than-life characters thrown in). In addition to the change in overall format, there have also been numerous individual characters sprinkled in here and there to excite a fan of the comics. Off the top of my head, we've gotten Nick Fury, Maria Hill, Jasper Sitwell, Sif, Peggy Carter and a couple Howling Commandoes visiting from the movies, as well as Deathlok and Glen Talbot (both with significant recurring roles), and Mockingbird soon to come, in addition to villains like the Absorbing Man, Lorelei and Blizzard.
I'm sad to see that the ratings aren't picking up at all, because I personally believe that Agents of SHIELD will only continue to get bigger and broader and introduce more and more characters and concepts from the comics the longer it's allowed to last. I feel that too many fans of the MCU gave up on it too quickly because it wasn't exactly what they wanted it to be the moment it came on the air, and forgot that a lot of fantastic genre shows (Buffy the Vampire Slayer, Star Trek: The Next Generation, Stargate SG-1, X-Files, and Chuck, just off the top of my head), had very rough starts. This show's start wasn't any rougher than those, and its upward trajectory at this point is just as strong. It would be a great shame to see all its potential be wasted in cancellation because fans were demanding too much too soon.