When TSR released the Dungeons & Dragons role playing game in the early 1970s, they created a new mode of gaming the role playing game. What is interesting is that they failed to rapidly follow up the success of their "fantasy" themed role playing game with a succession of game releases in other genres. While many of the first role playing games were shallow imitations of D&D...some were even Vacuous to use Gygax's terminology, it was other companies who first entered the marketplace with non-fantasy RPGs.
It wasn't long after the publication of D&D that Ken St. Andre drafted a set of rules for a science fiction themed role playing game entitled Starfaring, and Marc Miller published Traveller in 1977. Where Starfaring was whimsical, and is a quintessentially 70s artifact that feels a bit like John Carpenter's Dark Star the rpg, Marc Miller's Traveller set the standard for science fiction rpgs. In fact, Traveller truly set the standard for any rpg product line that was going to compete in the rpg marketplace. Marc Miller's creation had a large following among the Space Gamer readership, and the publication of support materials for the game led to the growth of FASA -- one of the classic old RPG companies. Traveller's success extends to the present, and Marc Miller currently has a Kickstarter campaign that promises a new edition that harkens to the old version.
Even though Traveller established science fiction as a viable genre for role playing games, it took TSR five years after the release of Traveller before they released their SF entry into the RPG marketplace, the Star Frontiers game. When Star Frontiers came out, there were those who tried to compare it to Traveller, but I have always felt that the comparisons were slightly off base as they represent different kinds of SF. Traveller's rules and back story, as well as the overwhelming influence D&D had on the early RPG market, gave the game a specific feel. Characters created in the game are typically former military who are now retired, or as James Maliszewski has pointed out a good many were former Mercenaries. Traveller campaigns had narratives along the lines of the Firefly television show, though it would be more chronologically accurate to say that Firefly has a Traveller feel to it. Traveller's own backstory was heavily influenced by Asimov's Foundation series with it's dying empire. Traveller campaigns were often gritty SF adventures filled with mercenaries and retired Imperial Officers spanning the Spinward Marches in pursuit of wealth and notoriety.
The D&D influence could also be seen in many Traveller campaigns, where players essentially wandered around the galaxy as pirates raiding Imperial space ships for their loot. This isn't to say that all Traveller campaigns were "spacey dungeon crawls," the official adventures certainly weren't, just that some people played it that way.
The science fiction background of Star Frontiers was quite different from that of Traveller. Where Traveller took place in a galaxy dominated by an interstellar Empire in a fairly settled area of the galaxy, Star Frontiers took place on the Frontier of civilization where a major corporation "Pan Galactic Corporation" -- later multiple corporations -- was sponsoring the exploration and attempting to profit. The Pan Galactic Corporation had come into existence to promote exploration and trade among four major alien races -- Human, Vrusk, Dralasite, and Yazirian. These races have only just begun to interact with one another, and have banded together on the Frontier of explored space. At that Frontier, they soon discover a new enemy...an enemy that threatens to destroy any civilization that chooses to explore the Frontier. That enemy is the Sathar, a wormlike race with hypnotic powers on the edge of explored space. The exploring races have only recently completed their First Sathar War, during which they formed the United Planetary Federation, and are now having to deal with terrorist attacks and sabotage by agents of the Sathar...agents from among their own people. In response to the Sathar's new warfare strategies -- espionage and terrorism -- the UPF has formed the Star Law Rangers who track Sathar agents and attempt to foil their plots.
The universes of the Traveller rpg and the Star Frontiers rpg have parallels in history. One is of an empire in decline, the other is of mercantilism on the rise. The tones of the settings are very different, but so are the rules. Where Traveller characters are retired from former professions and already have a number of skills at which they are proficient -- especially if the characters were generated using the Mercenaries or High Guard supplements -- Star Frontiers characters are relatively inexperienced. Even in the Expanded Star Frontiers rules, the characters have training in only two major skills -- and that training is at the lowest level. The characters start near penniless and are in need of employment. Players can be thankful that the Star Law Rangers are always looking for recruits, that the corporations are always looking for someone willing to risk Sathar attack while exploring planets on the Frontier, and then there's always the possibility of playing a group of Sathar agents...
Star Frontiers is a game that has a background that is rich in ideas for development...but it is also a game where one has to dig in order to find these ideas. Trying to find out the history of the Star Frontiers universe is not an easy task. Prior to the publication of Zebulon's Guide to Frontier Space there was not a clear timeline of the development of civilization. One had to induct heavily from the introduction in the Basic Game rule book, read and reread the racial descriptions, and scour every module for minutiae to get a sense of what was going on. Zeb's Guide did some of the work for you, as it advanced the timeline to a point after the modules and to a point where the Sathar had developed mind controlling organisms that latch on to the victim's back to take over the nervous system (fans of Puppet Masters and Iron Empires take note). Taking the Frontier beyond an outline and into a fleshed out campaign setting takes time, but it is worth it.
I've read the rules many time, but have never actually played the game. It's an easy system, though I've recently come up with an even simpler version of their Basic Rule with my own Extremely Basic rules, but I might just use the setting and play the game with another game's rules set. Maybe d20 Modern/Future, they did write a Star Frontiers setting section for the d20 Future book and had a web expansion with stats for the Sathar, maybe Alternity, or Savage Worlds. Heck...I might just use the Traveller system for it, when I get my copy of the 5th edition. It's a great game too.