Traveller (in all it's iterations) was unique because character creation is part of the gameplay; from determining homeworld to the years spent in character's career all feeds into the character you end up playing. Traveller also was a first in that in some versions a character could fail to survive character generation! Also Traveller is a fairly "hard" science-fiction setting. Playing characters who were older and professionally trained and experienced was a new compared to other games I had played.
MegaTraveller completed the basic concepts presented in the initial version (the 'Little Black Box' as it is known) with a uniform Task system, detailed vehicle and ship construction rules and thorough star system and trade rules. MegaTraveller was so complete, you could almost play it solo. The world creation and trade systems didn't require any real character interaction, so one could spend hours rolling for what trade items your merchant could acquire and dice out the haggling over price. Some of the first programming and spreadsheets I created for my own use were for automating and supporting MegaTraveller gaming.
On the downside, MegaTraveller is a very simulationist kind of game. Starship combat, for example, was quite complex (I didn't quite grok it until I read an article in Challenge magazine that explained it). Vehicle construction required a spreadsheet to keep track of details. (The next edition, Traveller: The New Era, was an order-of-magnitude more complicated!)
Also, MegaTraveller doesn't have much in the way of character advancement systems. On the one hand, this made sense: It took your character forty years in the Navy to learn how to navigate as good as he can. What makes you think only a few game-months are going to make him any better? Character advancement is measured more in the story successes your character(s) achieved. Unfortunately, aside from purely financial gains, there is no mechanical way to measure such successes.
Traveller also benefits from a very creative and active fan community. Massive amounts of history, backstory and additional materials have been written for it in all it's iterations. The Third Imperium (the default setting area for Traveller) had over eleven thousand star systems, and that's just the core setting!
So what can we learn from MegaTraveller?
- Character creation that gives your character backstory is a good thing.
- A unified, but very flexible Tasking system means you can handle just about any challenge.
- Well developed 'sub-games' give the DM and players a lot of easy tools to use.
- Character Advancement needs to be measurable, however. Even social and political gains need some way to quantify.
- Broad settings allow for players to create a lot of details. and even have considerable impact, without changing the setting as a whole.