In the course of my re-reading of Gamma World, I was reminded of something besides the appropriateness of Larry Elmore's art, namely how little characters improve mechanically over time. First edition, unlike second, does include an experience system (2e has a status system, which is a bit different, as I'll discuss shortly). Characters gained XP equal to the hit points of foes defeated, the value of artifacts discovered, and for "outstanding actions," as determined by the referee. Under this system, advancement is slow, slower than in OD&D, even after the Supplement I XP rules were introduced. There are no levels as such, although there are XP thresholds that grant a random 1D10 roll on the "experience bonus matrix," resulting in either a +1 increase to an ability score or bonuses to hit or damage in combat.
Since the benefits of gaining XP are random and small, Gamma World characters, much like Traveller characters, don't really change all that much after initial generation. This is because, with a few exceptions, ability scores have only small mechanical effect, so increasing them won't have a huge impact on gameplay. Interestingly, the game's second edition made "experience" even less important, eliminating the bonus matrix and adopting a system of "status points." Status points were awarded for roughly the same actions as XP under 1e -- the thresholds are even similar -- but each threshold grants status ranks that modify one's Charisma when dealing with NPCs. High status characters got better reactions from others and could attract a larger and more loyal body of followers. Interestingly, even 1e includes lots (in a relative sense) of rules relating to Charisma, making it far from a dump stat in Gamma World.
Related to all of this is the fact that combat success in Gamma World was governed by "weapon classes" rather than character levels. Each weapon had a weapon class that reflected, I suppose, the ease with which one could score a hit with it in combat. Consequently, high tech weapons had better weapon classes than more primitive arms. One's ability to hit an opponent was almost entirely determined by the weapon class of the weapon one wielded and, with some small exceptions, it didn't improve much. Thus, most characters were as good at hitting an opponent with a Mark V Blaster at the start of their adventuring career as they were many sessions later. It's a system that seems to have its origins in Chainmail, although others can correct me if I'm mistaken on this score.
In any case, it's fascinating to look back on Gamma World and see a game that, for all the obvious influence of Dungeons & Dragons upon it, opted to make experience of comparatively small importance. Character "improvement" in the game is not measured primarily by increased combat effectiveness or the acquisition of new abilities but by greater engagement with and influence on the game's setting. This is made even more explicit in the second edition, which, intriguingly, elevates "status" to a much higher degree of importance. Both first and second edition make much of reaction rolls and NPC retainers. To my mind, this suggests that the "goal" of these games, to the extent there is one, is tied up in an endgame about which neither the rules nor the referee's advice says much: the establishment of a stronghold/power center, complete with followers.
It's something my friends and I must have picked up on back in the day, because I distinctly recall two different Gamma World campaigns that became increasingly involved in "power politics." In the first, a pure strain human PC joined the Knights of Genetic Purity and used his personal charisma and stocks of Ancient artifacts to claw his way up to heights of power within the cryptic alliance -- until an unfortunate encounter with high intensity radiation resulted in his becoming a mutant (this was under the 1e rules before PSHs were explicitly ruled immune to radiation) and thus falling from grace. In the second, a mutant animal (a frog, as I recall) created a rival power structure to the Ranks of the Fit, attracting many to his banner, under which he hoped to unite the region and establish an empire.
I'm going to continue re-reading Gamma World closely to see what other oddities I notice. I'd forgotten more of it than I'd realized and the process of rediscovery is enlightening.