The first science fiction roleplaying game is generally thought to be Metamorphosis Alpha, not Classic Traveller. MA was the first RPG I even bought. Comparing the two is interesting. Metamorphosis Alpha is clearly in the vein of D&D which is not surprising as they were produced by the same company. Also D&D was pretty much the only game in town, if you pardon the pun. The first law of success is to copy someone who is already successful. Unlike D&D MA had no experience system, levels or classes. Instead characters were differentiated by mutations mostly and to a lesser extent stuff they found.
Then Traveller came along. It definitively broke the D&D mold. Starships and Spacemen was published a year after Traveller and retained several D&D conventions such as classes and levels (which seemed to work pretty well.) But Classic Traveller avoided classes, levels. It also had no comprehensive experience system. Characters came to the game fully formed. In this the two games were similar.
From there on the two games were completely different in tone, setting and approach. Travellers were given a universe to move about in. MA characters were given a lost derelict generation ship to explore and live on for however long the ship's systems kept working. In Traveller the universe would keep on going, evolving, and changing. In MA the ship would eventually wind down. The shiny gear you'd find or loot would eventually run out too. Despite this Alpha had a surreal and positive tone (when a sentient bush with a laser rifle wasn't shooting at your feet and telling you to dance telepathically.)
One fact about Traveller blew my mind after dealing with laser swords and protein disruptors. Despite the high tech everywhere you generally used guns firing bullets that no one would have trouble finding in a chain store to day anywhere south of Maryland and west of Jersey. All the technology was pretty accessible to us TL 7 types.
Traveller had a more mature tone. It even gave you equations with square roots! It also clearly displayed its designers' wargaming roots (far more so than D&D in its finished form.) Looking at the core LBBs they almost seem like a collection of minigames: survival (in your career), trading, combat. Some people refer to Old School games as a collection of unrelated subsystems. To a degree this is true of CT. The beauty is in how minimal and engaging the rules are. You also have to give GDW points for using all d6's especially if you were a broke kid in the 70's and couldn't get to an FLGS.