Traveller, to me, is (in no particular order):
- No levels. (There are effectively classes, but we call them careers.)
- 2d6 is the predominant mechanism.
- Dice only have six sides.
- Skills based system.
- Low granularity. (Medical-3 is a full doctor.) Skill 5 probably means you are the best currently alive.
- FTL travel, but no FTL comms. You have to carry the message out of a system to get it anywhere else.
- Jump drive. Max of jump-6. Takes a week to get there.
- Ubiquitous anti-grav. (Air/rafts!)
- Most guns still use bullets in some form.
- Illogically distributed populations and technologies.
- 2D space.
- Battledress.
- Swords in space.
- Ship mechanics are based on volume, not necessarily mass.
- 100 diameters.
- Human centric, but lots of "types" of humans. Plenty of aliens, though.
- Humans are recognizably human. No transhumanism; no machine intelligence; surprisingly minimal automation.
- Psionics exist, but there is no other overt magic.
Note that some of these have setting implications, but there is no implied setting beyond that.