Of all the things that Gamma World does right, perhaps the most significant is the addition of cryptic alliances. Described as "secret or semi-secret organizations," cryptic alliances are, I contend, a remarkable design innovation that is generally unsung. For what cryptic alliances do is provide, without the need for game mechanical support, an easy -- and flavorful -- means for players, through their characters, to become actively involved in the development of the game world. All the cryptic alliances have ideologies and agendas and joining one (or more) of them immediately embroils the PCs in their machinations, whether they like it or not.
Now, it's true that the rulebook does not specifically mention the possibility of joining cryptic alliances, so I suppose it's possible that some referees didn't allow for it, but I did. It seemed obvious to me that the joining -- or opposing -- the cryptic alliances was one of the major elements of campaign play in Gamma World, a supposition supported by the second edition of the game, which was very explicit on this point. In my campaigns of yore, I vividly recall characters who joined the Knights of Genetic Purity, the Restorationists, and the Ranks of the Fit, and I'm sure there were others I've since forgotten. In each case, the campaigns quickly acquired a sense of direction and, occasionally, urgency that they'd lacked before, when the PCs were rootless wanderers rooting around in ruins of the Ancients for high-tech artifacts.
I've bemoaned the loss of D&D's endgame previously, but the simple truth is that that loss is self-inflicted to a great extent, because D&D has done a consistently poor job of providing any structure for its endgame. Gamma World is no better when it comes to mechanical support, but it's my contention that the introduction of cryptic alliances does something much more needed by showing not just what high-level characters do but also why. I mean, it's all well and good to say that high-level D&D characters settle down and rule baronies or even -- *shudder* -- become gods, but why?
Cryptic alliances offers answers to such questions. If you're a Knight of Genetic Purity, you've got a long-term goal: to cleanse the world of mutants. If you're a Restorationist, your goal is to rebuild a newer and better society from the ashes of the old. How your character goes about doing that is left wide open, maybe too open for some, but, for my money, the fact that Gamma World provides any structure whatsoever for what a high-level campaign might look like is a point in its favor. For that matter, this structure is equally applicable to low-level campaigns and its open-endedness makes it very easy to use, regardless of who the PCs are.
The legitimate gripe about the cryptic alliances is that, of the thirteen provided, a goodly number of them are either inappropriate or impractical for the characters to join. Likewise, many of those that could be joined are primarily militaristic in their goals, which limits the types of campaigns that could be structured around them. Still, I can't help but be impressed with cryptic alliances as a concept. I think most RPGs would benefit from the existence of in-game organizations that provide the PCs with goals and belief systems to latch on to and they're equally useful to referees. That's why I'm all the more surprised that it wasn't until Paranoia that we really began to see the promise of the cryptic alliance concept fulfilled and it would still be many more years before it became a pillar of roleplaying design.