So, if D&D was focused on free form creativity placed completely in the hands of the players and DM initially, what did it become? And did any of this element stay with D&D over time?
Evidently the advent of AD&D was born out of several needs. First there were issues involving Dave Arneson and Gary Gygax that related to lawsuits and the like. Gary needed a game that was distinct from what had come before. Even though the original rules were formalized by Gary much of the initial work was forged by Dave. I personally see the work as a synergy, but that's just personal commentary from the outside. Whatever the case, lawsuits were one reason Gary needed to create a distinct game. This had to do with intellectual property rights. But I really don't see the problem other than maybe Gary and Dave didn't get along. Or maybe Gary saw the game as his or Dave did. Hard to say. And I'm not sure exactly how this worked, because the original game was published through like 1979 when AD&D was finished, but whatever.
Second, there was the revelation provided by Rob Kuntz that AD&D was a response to the commercial opportunity to market supplements, modules, campaigns and the like to a hungry and very eager audience. Rob mentions this idea being spawned by the receipt of a submission module. But there seems to be other indications that Gary already was aware that people wanted content. Greyhawk was the first, then Blackmoor and shortly after Eldritch Wizardry, Gods, DemiGods and Heroes and Swords & Spells. Strategic Review and Dragon Magazine were also providing supplemental info since 1976 or so. The disconnect for me is that like most human decision this was a complex one. Could be that the module submission was the first time they really considered module creation, but the idea of creating content seemed to be a part of D&D early on.
So how and in what way was AD&D different? Well, first of all we get a complete rewrite of the rules, additions in numerous areas; more detailed class descriptions; addition of optional rules like psionics & the bard; exposition of basic cosmology of the D&D universe; lots of design helps for dungeons, wilderness, aerial, and campaign types. Combat was kept basically the same, but the charts changed and lots of additional rules were added. We get quite a bit of prose about how the D&D universe works in terms of basic physics, magic and the like. We also get a bigger and still strongly Greyhawk flavored list of magic items, and spells. The rules are presented a little more organized than the original books. The game is basically the same in theory only more detailed, complex and content rich.
The effect for me is that AD&D is a more limited fantasy game in scope. But only slightly so. What I mean by this is that the realm of AD&D is understood to be a Medieval European Fantasy setting with some of the universal structure inherent in the game. You can opt of just about everything: the Greyhawk flavored elements, optional rules, additional combat rules, the cosmology. Pretty soon you are playing a fairly basic game. But you do this by cutting away from the game. We didn't play this way. We used all the magic items, artifacts, cosmology charts, tables and on and on. We didn't use weapon speed, but just about everything else was added in the mix. Our game was sort of the standard AD&D world. Fairly well written out, explicated and contained in the 3 main AD&D books.
We did create dungeons, campaigns, npcs, worlds, magic items, and other stuff. So why was what we did different from what 0e people did? Well, the difference was in terms of scope in my opinion. AD&D implied a sort of carbon copy series of worlds that all felt pretty much the same. Bigby's Crushing Hand, Mordenkainen's Disjunction, Rary's Mnemonic Enhancer, Baba Yaga's Hut, The Hand of Vecna were all there regardless of what dimension your world might exist within. Even when you left those elements out a D&D world carried a sort of default definition from AD&D on. Now it didn't have to be this way, and I have talked with others who did it differently. It wasn't until much later in my gaming career that I began to step outside of that box.
Personally I think AD&D reinforces that kind of narrowly defined play. That's not necessarily a fault, and it certainly doesn't proscribe other types of play using the system. Gary himself makes it clear that cross genre games are possible and TSR has produced for your gaming pleasure: Boot Hill, Top Secret, Metamorphosis Alpha, Gamma World ... well you get the idea. Gary also speaks at some length about designing your own adventures and campaigns. So this model still existed within AD&D, but the game itself was fairly narrowly defined within the classic high fantasy route. I also know that there were many that continued the old approach to gaming and took AD&D apart and reassembled it to their liking. I suppose, for me and my early gaming friends, we were accepting the game RAW as the model designed to play. Straying outside that box was only to be performed with caution. An idea Gary himself reinforced within the DMG and his later book RPG Mastery. This was a clear departure from the earlier approach.
Subsequent editions increased this divide until we get to DnD 4e where rules for balancing encounters and constructing adventures, properly disseminating treasure value and XP are so tightly constructed that it is clear you may only create within a narrow tunnel of design. Now, technically nothing prohibits you from straying outside this structure. You can still do just about everything you want, but try and change core mechanics or structure and you usually get rebellion. Take powers out of DnD for instance and players often complain or refuse to play. Limit race and class selection and grumbles, groans and outright refusals are received. The game is what the game is, don't screw with it. Try and amp up an encounter or throw something in unexpected and 4e gamers chide you for not balancing the game. Can't be too hard, or too easy. Has to be "just right". You can try to change things, but the ethos works strongly against the free wheeling, creative anything goes days of 0e.
Now, if you can find a group willing to allow you free creative license with whatever version you play then great. Like minds make for progress. But why play a game where most players don't like that appraoch? Wouldn't it be better to play a game where everyone understands that they are entering an unknown and unexplored region of the imaginative landscape? Where the familiar tropes and landscapes are left behind as they sally forth towards a realm of high strangeness and unexpected danger. A land where you can truly let your imagination soar.