The prison facility dubbed The Kyln may not bear much resemblance to the version seen in the comics (functioning primarily as a power plant, while also housing inmates), but the nod to the original version won't be missed by fans. Especially since Star-Lord did spend a brief time within its walls.
Little time is actually spent in the film to establish the ranks or history of the Xandarian Nova Corps, but the two most memorable members of the force aren't simply tossed in haphazardly. First, there is Rhommann Dey (John C. Reilly), who comic book readers will know as one of the few remaining Nova Corps members of Xandar who inducted Richard Rider into its ranks.
Besides him stands Garthan Saal, who was driven mad with power and anger after Nebula destroyed the planet's population. Neither characters gets their full story played out in the film (probably for the best), but it's a nice nod to the fans regardless.
It's no secret that Marvel has shaped several of their films around the age-old convention known as 'The MacGuffin' - an object that bears extreme significance, and drives the events of the film almost on its own. The debate of what does or doesn't constitute a MacGuffin is never-ending, but even Marvel's Kevin Fegie has admitted that the term applies to the many 'Infinity Stones' scattered throughout Phase 1 and 2 of the Marvel universe.
With Guardians, we get yet another item in the form of the purple gem sought by Thanos and Ronan (it's not what the thing does that matters, just that someone wants it). But instead of ignoring the trope, Star-Lord himself describes the item as possessing "a real Ark of the Covenant, Maltese Falcon vibe" - two films driven almost entirely by MacGuffins.
Lloyd Kaufman Cameo
As a longtime idol of James Gunn, director/writer/producer and co-founder of classic horror studio Troma Entertainment Lloyd Kaufman is given a blink-and-you'll-miss-it cameo in the film's sequence in The Kyln. When the Guardians first enter the prison, a shot reveals a number of inmates angrily shouting at Gamora - with Kaufman unmistakable alongside them.
Nathan Fillion Cameo
The working relationship and collaboration between James Gunn and Nathan Fillion led many to suspect that the Firefly alum would appear in a key role (with some even claiming he would appear as superhero Nova). Gunn debunked that rumor himself, but confirmed that Fillion would have a cameo part. Fillion's cryptic claim that those looking for his appearance should "check the credits" implied a voice-over role, and that is the case: he's the large blue alien in the Kyln that expresses his desire to "lather up" Peter Quill in gravy and "go to town."
The Other Returns
Once again, those wishing to serve Thanos are forced to endure the verbal assaults of the Mad Titan's minion, known simply as The Other (played by Alexis Denisof). Apparently, Ronan the Accuser (Lee Pace) is even less patient than Loki, as he ends The Other's life once and for all.
If there's on thing we've learned from great fiction, it's that a great ruler needs a throne just as impressive. It's hard to imagine Thanos from the comics not seated atop his throne, grinning ear to ear, and the film version follows the same idea. While the throne was only glimpsed at the end of The Avengers, Guardians gives a full view of the (hovering?) chair, with Thanos looking as if he was recreated inch by inch off of the comic book page.
Giving Tree/Ranger Rick
The fact that Peter Quill left Earth behind in 1988 is a source of several jokes throughout the film, since his knowledge of pop culture effectively ended with Alf, Footloose, and Raiders of the Lost Ark. Unfortunately, that also means that two of the more inspired jokes of the film may slip by younger viewers completely.
We speak, of course, of Quill's nicknames for his non-humanoid comrades. First, he refers to Groot as "Giving Tree," alluding to the children's book by Shel Silverstein. But the best nod to his childhood comes when he refers to a bomb-building Rocket as "Ranger Rick" - the name of a children's nature magazine first published in 1967. Aside from the magazine, Rick was also the star of illustrated short stories, revealing him to be an anthropomorphized raccoon.
To delve into the dark underbelly of galactic crime, the Guardians must travel to a mining operation dubbed 'Knowhere' - explained by Gamora to be the skull of a long-dead, enormous Celestial being. In the comics, the same origin story is carried forward. As out of control as life on the station may seem, it would become the base of operations for the Guardians, claimed to be resting on the very outskirts of the known universe.
It was expected that Guardians would blow Marvel's Cosmic universe wide open, and the mere presence of Knowhere confirms it; or rather, the presence of the severed head of a Celestial. For those unfamiliar with the race of cosmic beings in Marvel Comics mythology, the Celestials are mysterious, armored, all-powerful beings who have made their influence known on human beings especially.
Having helped engineer human beings a million years in the past, the Celestial tampering planted seeds for more than a few important story arcs in Marvel's history. We won't go into too many details, but the existence of the Deviants, the Inhumans, and strong ties to the Kree and Skrull war are just a few (during the Collector's explanation of the Infinity Stones, fans can even glimpse what looks to be a Celestial in the flesh).
It's hard to say if Marvel will leap to those stories any time soon, but the presence of Knowhere provides a strong hint.
Ophelia Lovibond's assistant may seem like little more than a background character, but her relationship to the Collector (and Marvel Comics) is a bit more complicated. Officially credited as 'Carina,' it's safe to assume she is Carina Walters, daughter of the Collector, and future wife of Michael Korvac (the villain known as 'The Enemy').