Everything.
Seriously though, what comes to my mind:
* in Traveller, you do almost all rolls with 2d6 instead of a d20. This gives you a different probability distribution. You usuall roll to pass an 8 - everything above that improves the result, with +6 (6 above whatever you have to beat) being a critical success; everything less than that may have negative consequences and -6 (failing the roll by six points) is a critical fail.
* Traveller has a vastly different class system. You can make your character older and for every four years "passed", you can roll on a table of life events, which may include good things (you get a politically powerful ally) or bad things (you lose a limb or an eye).
* It is also very low-magic (at least the setting we are playing.) Psionics exist, few and far between - they have "classical" mind skills lile telekinesis, telepathy etc. Otherwise, not much.
I am not too sure what is due to our setting and what is due to the system itself. We are playing in a Star Trek-like world, 5000 years in the future. Thus, the feeling is different a lot.