Someone on Facebook just asked for recommendations for a "space opera Star Trek" game, and of course there were all the usual suggestions (Fate, Savage Worlds, etc.)...and of course one large contingent recommended Traveller. I went a little ballistic, and it reminded me that I don't think I've weighed in on Traveller in this blog yet. So here's what I said. *** Would people please stop recommending Traveller to newcomers? If you go into your space opera game expecting blaster pistols, derring-do, laser swords, force fields and the fastest ship in the galaxy (to say nothing of teleporters), you'll be bouncing off Traveller at every turn: it's a low-tech shotguns-in-space world with high lethality, low heroics, and a hard maximum speed for all starships. In fact, the idea of even owning a starship--the core element of the genre--is a distant, mortgage-strangled dream. Traveller is very very good for emulating low-tech sci fi like Alien, or hard sci fi like The Expanse. But if you want Star Wars, Star Trek, Buck Rogers, Guardians of the Galaxy, Dark Matter, or Mass Effect, you'll probably want a system where PCs can take a few hits and where players don't start the game as ex-military pensioners in their 40s who have almost no way of improving their skills.
In my experience, generic systems (GURPS, Fate, Savage Worlds, d6) tend to be the best and cleanest at doing the sci fi YOU want to do, while other systems (not just Traveller, but Ashen Stars, Coriolis, Mutant Chronicles, Fragged Empire, etc.) tend to be good at expressing THEIR world, but not so much yours. (Generic systems also tend to be skill based and let players improve incrementally, which is very much true to the genre.)
EXCEPTION: There have been a number of Star Trek games which have usually managed to get the genre right, at least (you can't have Star Trek without phasers, shields, and transporters!); and most of the indie/OSR space games I'm aware of (Stars Without Number, White Star, Solar Blades & Cosmic Spells) are designed to be modular, with the GM adding whatever rules they need to regarding aliens, psionics, and ancient alien tech. But anything with a D&D base is probably going to involve levelling up according to preset classes, so you have to be ready for that.
(ADDITIONAL WARNING: If you want a simple Powered by the Apocalypse chassis, Uncharted Worlds has a surprisingly abstract combat system, and you might find Offworlders more direct and satisfying.)
I love Traveller, and goodness knows there's a lot to love. But a LESS generic, harder-to-hack game has never been invented. Its unusual premises are baked right into the character generation, where you don't know what skills you're getting, you're "mustering out" of a "career" in middle age, and everyone has "social status" and "education" but not charm or intelligence. If you don't use Traveller to simply play Traveller, you're going to fight the system at every turn.