when people who have played the leveling-up games try to play traveller they are very much taken back at the thought that their characters are semi-fixed, and are a bit puzzled as to how they are to proceed. "well then what are we doing?" is the usual response. the answer "whatever you want" either attracts them (some) or demotivates them (most).
This is an important point for whichever edition one chooses. "Whatever you want" can be especially crippling if the Players a) have little idea of what the setting is like; b) have little idea of what options are before them.
I've read about many
Traveller games that get set up but never continue after character creation, and in many cases it is because the Players have no idea what the could be doing, let alone know that they should be choosing what they are doing.
I think a solid talk before -- either before or after making characters -- is vital when creating characters in
Traveller about goals. As flykiller rightly points out, it's about interacting with the environment, but without some sort of agenda it might well fizzle. D&D provides the agenda through the aquistion of XP.
But as the last page of Book 3 (1981) states:
The players themselves have a burden almost equal to that of the referee: they must move, act, travel in search of their own goals. The typical methods used in life by 20th century Terrans (thrift, dedication, and hard work) do not work in Traveller; instead, travellers must boldly plan and execute daring schemes for the acquisition of wealth and power.
"In search of their own goals" is, I think, vital for the open-ended, sandbox style play that I think
Traveller was designed to handle. But, again, if the Players don't know much about the setting or what they can pursue, all that will be lost.
Having a discussion a bout this issue means everyone can get on the same page about this matter. What do the characters want? Why are they risking their lives traveling the stars? Are they out to build a fortune? Take vengeance on someone? Build a trade empire? Topple a government?
All sorts of adventures can feed into any of those goals (and more) -- either through the money gained in doing high risk/high reward jobs that will provide resources for moving forward. Or each adventure can be steps along up the rungs of the ladder toward the main goal.
But if the Players know what they are trying to reach -- especially if they have chosen it -- they will own those goals and be committed to them much than if the Referee tries to snag them with "hooks."
Moreover, if the Players and Referee have a talk about the goals the Players want to pursue with their PCs, the Referee can then, within the setting he has created, select the opportunities and obstacles for the PCs that both feed the Player's goals but also excite him as things about the setting he wants to share.