A few months back, I wrote about the original Dungeons & Dragons rules. To be clear, I wasn't reviewing the game from a play point of view. That's been done many times over the years. Instead, I examined the three booklets for insight on how they presented the rules, and what that told us about how the game was played and presaged what was to come. As someone who's been tasked with writing RPG rules myself, it's interesting to see how it was done at the genesis of the hobby.
Just as interesting, however, is the dramatic evolution in the early products that followed. When we look at the various supplements (Greyhawk, Blackmoor, and Eldritch Wizardry) we get a glimpse of how the game's play was changing and how the rules had to (quickly) adapt. As any experienced designer will tell you, how you write the game rules shapes gameplay, but it's gameplay that shapes how you write supplements.
The most obvious thing the three books present is the new "stuff:" New classes, new spells, new monsters, and so on. Obviously, the driving force behind these books is a simple player demand: More! Give us more!
Among its many innovations, for example, Supplement I: Greyhawk introduces thieves. This is significant because we suddenly see that players want to do more than just fight monsters and cast spells. Without a doubt, before the advent of the thief class people were sneaking up on monsters, picking locks, and climbing walls, but now these tasks were codified by creating a specialist that excels in them.
Inadvertently, of course, what these new rules also do is suggest that it's impossible for non-thieves to attempt these tasks. By adding options, others are removed. Before, if a player wanted his character to climb something, the DM just winged it, perhaps not worrying about rules for success or failure at all. Now the thief has a percentage chance to climb a wall, but the fighting man has no such score, implying that he can't even try. Definition leads to limitation, but this is a lesson not learned for a long time. For now, D&D players who got their hands on Supplement I were thrilled.
New classes and features were introduced to the game at a frenzied pace. With all the possibilities available in the game already, it's hard to imagine that by the time Eldritch Wizardry came out people were bored with all the spells and magic items. So why introduce psionics, which layers in a whole new way for PCs to have special abilities?
It was because of all the other options available. As the amount of content grew, it became easier for players to replicate the fantasy and science fiction of the time. Psionics were likely introduced not because they were good for the game, but because they existed in so many sources outside of it. D&D was quickly becoming a way to emulate fantasy fiction that hadn't existed before.
Further, the spell, monster, and magic item descriptions change in Supplement I and exacerbate with Supplement II and III. Each of these aspects are presented with a greater level of detail and a more elaborate explanation in each release. Consider the single sentence that the gorgon receives in Monsters & Treasure with the virtual treatise on sahuagin given in Blackmoor.
We also start to see more and more notes on how one spell might interact with another spell or magic item. The designers, obviously, began to realize that they were creating a vast matrix of abilities and powers that not only changed reality, but had to exist within a reality that would change again the very next round. Nothing could be added to such a game without first considering the effect it might have on all other aspects of the game. Could the effect be dispelled? What happens in area of antimagic? Does it work on magic materials?
So great was this need for clarification and context that many of the spells and rules of the original books receive "additions and changes" in these supplements as well. Reading through them is like reading through a list of answers to questions you've never asked. Obviously, though, someone was asking these questions, and Gygax and company felt they needed to be answered.
Here, then, is a wonderful example of gameplay driving rules writing. Looking at all three of these books, you get the sense that the rules in them are not just part of a game that a lot of people are playing, but are pushing to its limit. It is, in its own way, invigorating. It makes you want to play the game.