FreeTrav said:
Q: Why should I invest my time and creative energy into Traveller, rather than WH40K or D&D?
A: because Traveller provides the opportunity to adventure in a world that's atmospherically closer to what we know than either D&D, with its dungeons and enforced "Hero's Journey", or the stark, unremitting grimness of the WH40K universe, while providing a broad canvas with a well-defined and believable background with which the players can engage on many levels. It can provide ultimate freedom, because the background is predicated on light regulation, but there are structures within the background that can provide a more directed game experience if that's the group's desire.
What are the tropes that Traveller plays to? What makes them preferable to other tropes?
Freedom of action: you can go anywhere. Sometimes there will be reasons not to and people who want to stop you, but you can go nevertheless if you have the travel means.
Personal reliance: you can't call in reinforcements or ask for clarification of your mission parameters from off-world.
The Unknown: all news from off planet is at least a week out of date. It is known that the scout surveys are more doillie than fig leaf for the IISS...
People are people: even most of the commonly-encountered aliens have recognisable psychologies and comprehensible biologies. Supply and demand interact in recognisable ways...
Stuff is stuff: really the only concessions to "breaking the laws of physics" are FTL travel and manipulation of Gravity. There's assumption about some materials science and abttery power-density improvements built-in, but the basis of the tech is pretty much "hard science" rather than "sub-ether rays" or "tech as magic". Chemical-powered slug throwers still have their place.
Edit: thought of another one...
You're an adult: you start play, usually as a fully grown adult with a decent skill set, rather than some wimpy kid clawing themselves up out of inadequacy.
For me, it's the package of them all that's attractive.
How do I show the new player who is drawn by those tropes that Traveller plays to them?
Have 'em skim through the basic rules. If you're not planning to use them, have them have a look at Freelance Traveller.