A post by Omer Joel on Classic Traveller got me thinking about it yet again (for the the record I've also been thinking about White Star and Operation White Box). As I see it there are two major reasons for its lasting appeal.
If you look carefully CT is a series of mini games. Character generation is the first. You risk your character's survival against building an array of skills. Will you character be over the hill as age drains vital characteristics? The trading game is essential to making your mortgage payments. Doing your homework here can be a life saver. Tip: Find a world where you can buy something low next to one where you can sell it high. Get rich.
Combat is fast and deadly. Do you remain undercover or advance? Do you carry ammo or smoke grenades to provide cover? Do you expend all your endurance striking powerful blows and running or let the other guys wear themselves down? The resource allocation and decision making comes up everywhere in Classic Traveller and the decisions matter. That was what we called immersion in the old days.
Artists and designers are familiar with the concept of negative space (it's not a misjump, no). Simply put, what you leave blank is as important as what you show. In Classic Traveller's case the setting was left blank aside from some assumptions you could draw from the rules. We knew psionics could get you lynched, there were nobles, and paying off your ship was generally speaking a bitch. That was about it. No Empire/Republic/Imperium/Federation setting taking up pages. No alien bestiary. Oh, the Animal Encounter rules were awesome too! You rolled up animals as you needed them and filled in the fur, scales, teeth, tentacles you wanted. This is a very different approach from many newer games.
Traveller didn't have a setting. You had to make your own and people did. Some friends went for an anime style universe. I went for a hard and gritty techno thriller approach. Another went completely gonzo. It all worked though. In fact we were starting to talk about merging all our games into one galaxy and take turns running stuff.
Once you marry your rules to a setting you restrict what people will do, in many cases this is unintentional. The designer wants to help new players start up. The problem is that can influence new players far too much. Games with an official setting are sort of like Ikea. Classic Traveller was more Home(world) Depot. Obviously there is a huge market for setting material (that fact astounded the guys at GDW initially). But people lose sight of the fact, it's your universe and you can do whatever you want. In fact after you buy the setting material you can fiddle with that all you want. Have all three Imperiums existing at once. Have the Zhodani be humans infested by psionic parasites. Have the Ancients be active in some areas. Have giant robots. It's your universe and there's more than one way to have fun.