The only long term sci fi campaign I was ever involved with was a Star Wars (d6) campaign. I've played a lot of Traveller over the years, but my friends and I were more enamored with the mini-game that is character creation in CT than we were in the game itself. I've also played a lot of other sci fi games, none of which had the kind of pull that Star Wars did.
I must admit that one of the reasons that the campaign was so successful is the way it was run. The party had ties to the rebellion, and as a pseudo-military unit were sent on various missions. In the hands of a good Referee and willing players, this set-up can work very well; however, I also believe that the source material played large in the longevity of the campaign.
Firstly (and most importantly, in my opinion) religion is an integral and even central part of the Star Wars universe. As a Christian, I have some serious qualms about what George Lucas calls religion, but Star Wars cannot be Star Wars without the Force. This is a far cry from most science fiction (like Star Trek).
Secondly, Star Wars has more in common with fantasy literature than it does with sci fi. The characters are archetypes found in fantasy dressed up for space travel. The hero wields a sword and rescues a princess. We hear Obi Wan referred to as an old wizard. I could go on. In addition, just as fantasy normally does, Lucas borrowed heavily from mythology. He took very basic mythological and cultural themes, figures and tropes and recast them for a space opera. Thus, like fantasy, the Star Wars universe feels very comfortable.
Lastly, there is also one very important factor that I don't think many appreciate. Due to the geographic simplicity of the Star Wars universe, it lends itself to the fantasy sandbox style of play much more easily than Traveller or dozens of other sci fi games I've played over the years.
Let me explain. Tatooine is a desert planet. Yavin is a forest moon. Hoth is an ice planet, etc. These are akin to hexes on a hex map, where each hex indicates a different kind of terrain. When one needs to have a more detailed map of a particular section of said hex, it is easily done, but for the most part all one really needs to know is desert, forest, ice, etc. This might not seem very important, but I believe it is. Compared with the level of detail required of even the simple and abstract system used by Traveller to describe worlds, the scheme used by Star Wars makes universe creation no harder than creating a hex map for a fantasy campaign. In contrast, even in its relative simplicity, Traveller is rather quite intimidating. I firmly believe that one of the reasons my friends and I never got beyond an entertaining number of one-offs in Traveller is the fact that none of us had the confidence to pull a multi-world campaign off.
In other words, the more a science fiction RPG has in common with fantasy, the more playable it becomes.
2 days ago