Character-Driven
I'm shocked no one has mentioned this -- Traveller was (is?) the most character-driven role-playing game ever created. There is not a single moment in the game, ever, that the player can't ignore the plot hook, give the Referee the finger, and do something totally different. As Painandgreed mentioned, you could just keep travelling from system to system, forcing your referee to roll up new planets, encounters and trade goods, trying to keep one step ahead of the repo men. Or you could build yourself a little Striker merc unit and start carving out a personal fiefdom for yourself. Or you could turn outlaw pirate. Or you could go industrial espionage. Or you could turn traitor and sell secrets to the Zhodani. Or you could become Solamani terrorists. Or you could open up a starport bar somewhere and lay low keeping profits high while interstellar war wages around you. And you know what? Whatever you did, the Referee
couldn't stop you. Only death or poverty could.
Beyond that, it's a really well-balanced system that D&D only started paying attention to with 3rd Edition. Classic Traveller had a purpose for every single characteristic excepting,
maybe, endurance (but if you played with the Snapshot rules endurance suddenly became very important). You had to study your characteristics before choosing your military career, maximizing your probability of not just survival but advancement and mustering out benefits. And yes, you could die in character creation (it's practically blasphemy to think they did away with that in future editions) but that was another balancing factor -- do you go Scout, with its highly-desirable skill set (automatic pilot skill) and risk better than 50% death chance in return for the possibility of mustering out with a Scout Ship? Or do you go Merchant and re-enlist term after term after term hoping for promotion to Captain before being forced into retirement so you can muster out with a Tramp Freighter? Or do you go Army/Marine and figure you'll join a Merc unit when you get out? Or do you use your Social Status to move ahead in the Navy and figure you'll use you're awesome skill set to hire onto a choice position on a Cruise Liner or Bulk Freighter?
It all mattered; it wasn't just random.
And how did the Referee combat all this crazy freedom given to the characters? With tons of charts and tables to create environments on the fly. And with odds that were constantly stacked against those characters. So the characters don't want to do the adventure hook and just want to trade? Well guess what, sooner or later they're gonna fall behind on their payments and they're gonna be faced with jumping their bank loan or accepting that adventure hook, after all. So the characters' merc unit just rolled a choice assignment running security on a backwater world? Tough cookies that a Mega-Corporation has decided that planet is profitable and the characters were hired by an anti-free-trade political faction. So the characters don't want to get involved in the Zhodani-Imperium war and would rather just serve liquor at their local bar? Well, okay, in that case I guess the Referee's screwed.
And Traveller was deadly. Not just because a bullet could kill you, but because no matter how big and bad you were there was always
something in the universe that dwarfed you. You think that tricked out corsair is pretty kool, huh? Well meet Mr. Azhanti High Lightning Battlecruiser, tasked with clearing the system of pirates. Oops, did you think like Han Solo you could out-maneuver the big Star Destroyer? I'm sorry, in this game the big ships are faster than the little ones. So sorry. Would you like to role up a new character?