|The world is generally humanocentric, with non-humans relegated to the margins, which is why the protagonists often interact with them.|
James Maliszewski was talking about the sword & sorcery underpinnings of Dungeons & Dragons, but the same thing applies to the sci fi pulp Traveller grew out of, and for the same reason: it makes aliens alien. Something unusual and not everyday.
(Both posts are worth reading in their entirety, for other insights into the thinking behind this kind of game. Especially the comparisons between pulp adventure and the picaresque novel.)
Weapons: for the same reason, Traveller tends to minimize sci-fi energy weapons in favor of mundane guns. That way the contrast is heightened when the PCs actually encounter them. I think, for my Trailing Marches setting, I'm going to have only conventional firearms be common.
Spaceship-scale lasers are well-established (because sandcasters are about the coolest thing Traveller ever did, so I want to include them), but engineering an energy weapon down to the size of a man-portable firearm is pushing the envelope of existing tech. They don't get smaller than a large rifle or light MG (think M60, or those harness-mounted things from Aliens), and even those are rare. Like Jump 3+ ships, they can usually only be afforded by militaries and big corporates. Bad guys. When a Player Character obtains one, it'll feel like a big deal.
Blade Weapons & Spacebuckling: Space opera often features characters using swords and bladed weapons. Various ideas have been put forward to justify this, from Dune's personal shields to Star Wars' laser swords. There's been more than one thread at RPG.net on the subject. Here's the most recent, from which I lifted the justifications I'm going to use:
I'm tempted to include some science ficiton-y blade types, like the vibro-weapons from Star Frontiers, the neon-lighted crystal katanas from Cyberpunk 2020, etc.
Artificial Gravity & Antigravity: I want to limit gravity tech in some way. If an air raft can just float all the way up to orbit, then why would a starship ever land on a planet? I want starships to land, dammit. And need to be streamlined. Or carry streamlined shuttles. Those cylindrical boats and cutters are hereby space-only, they can't land on a planet.
Let's say grav vehicles have a maximum ceiling, and it's pretty low, maybe 10 meters or so. They can't go higher under their own power, and if airdropped by a starship (or if they drive over a cliff or something) they slowly sink to their ceiling altitude. They work by projecting a downward anti-tractor beam that pushes them up off the ground, so they do exert ground pressure, but it's spread over an area as wide as the vehicle, so there's less ground pressure per square inch than a comparable wheeled vehicle. Maybe equal to a tracked vehicle.
Atmospheric shuttles and atmo-capable startships are usually shaped like lifting bodies or a cross between a cargo plane and the NASA space shuttle (I wish my Tigers of Terra comics weren't in storage; I'd scan a picture of a Goliath). They have wheeled landing gear and make conventional runway landings and takeoffs as often as possible, to save wear and tear on the antigrav. Perhaps VTOL takeoffs and landings risk a breakdown in the same way as using unrefined fuel.
Also, I'm tickled by the idea of ships only having internal gravity in some sections, because it's expensive or uses up mass or something. So, like, you have gravity in the crew quarters and lounge, because sleeping and eating in zero-g is such a pain. But you don't really need gravity on the flight deck, right? I mean, you're strapped into a chair anyway....