There are a few guys that I think are making some very significant contributions to the way we are thinking in the more "modern" period of pre-3e D&D. Some of what these guys are saying is really applicable to any edition of D&D because they are observations about fantasy and swords & sorcery in general. Some of it is relatively specific to the older editions.
Before I mention the guys I'm talking about, a disclaimer. This post is not about publishers, and it's not about people who have contributed great house rules, and it's not about people who write adventures, etc. It's solely about people who are altering the thinking, or contributing new attitudes, etc. Indeed, these guys are probably rejuvenating ideas that have been around from early on in a seed form, but are putting them together or putting a new spin on things.
T Foster. Foster tends to analyze the underlying structure of the game, by which I mean things like the frequency of monster types, the importance of empty rooms, and so on. In particular, he is brilliant on the topic of large dungeons, how they can be used to best advantage by the designer and DM, etc. Foster has never written any really long discussions, being content to post on message boards, but his posts are often incisive to the point of providing real "aha" moments for the reader. At least for me, anyway. Instead of blazing new trails without much reference to what was powerful stuff from the "old days," Foster tends to be incredibly good at drawing out that powerful "old days" stuff and conveying it in a way that I, at least, didn't perceive. Things that appeared unrelated or unimportant are suddenly transformed into a coherent tool that can be used in new ways, or simply in ways that I never thought to use them.
Philotomy Jurament. Philotomy doesn't offer any general philosophy of gaming, and his thoughts are a scatter-gun of house rules for WhiteBox OD&D... but he explains the house rules in a way that cuts to the game's framework. Most people who write house rules are focused on the fantasy feel or the workability of the rule. Philotomy's thoughts on house rules are generally deeper than this. Here we are looking at a superb mosaic of thoughts about the structure of a game. When they are read together, they offer a broad range of ways to build WhiteBox D&D as a toolbox rather than as a ruleset. Lots of people talk about how this could be done - Philotomy provides specifics for doing it.
Kellri. Most people are familiar with Kellri's incredible book of tables for AD&D (or OSRIC). But what I'm talking about here is a "philosophy" that's also in many ways mirrored by James Raggi ... and these guys HATE each other in a serious way, too, which is funny. An observation that Kellri made somewhere is actually a really significant thought, if you take things in that direction. The paraphrase, since I can't find the actual quote, was something like "The DMG was like a Hustler magazine kept under the mattress." Kellri has a vision of D&D as a hallucinogenic world illustrated by Erol Otus, where sex is a constant underlying feature, like Heavy Metal magazine. It's weird and it's supercharged, and it's not safe for work, and it's not at all politically correct.
James Raggi. Jim and I happen to disagree on just about everything, by the way, but I think each of us eyeballs the other and sees the value of what we do. It's just that our approaches are quite different.
(a) Fantasy as tragedy. Jim is an American expat, but in many ways his approach to things is very Finnish (where he lives). I'm going to go out on a limb here by talking about a culture/mythology I'm only partly familiar with, but here goes anyway. Finnish mythology has an odd twist on it - just as Machiavelli said that every great action begins with a crime, Finnish mythology has a general sweep to it suggesting that epic greatness is not just built upon victories - it comprises betrayals, tragedies, and accidents as well. You can't make an omelette without breaking a few eggs, and Finnish heroes are tainted or haunted by the eggs they have broken, often unwittingly. It's much like the Ancient Greek concept as well, although in the Ancient Greek conception the tragedy of the hero is directly the result of the hero's actions, whereas in Finnish mythology the tragedies are more of a byproduct.
(b) Monsters as independent, unique beings. This is a powerful point if you are playing a game that "simulates" or takes place in a world based on sagas and epics rather than pulp S&S. In folklore, monsters are generally quite unique. I think this is a good point, but I also think that for gaming purposes a whole lot of the game needs to provide the players with more of a baseline level of knowledge in order to promote strategy.
(c) Gaming is an art. I totally disagree with this, and it's an idea that has been propounded for years. Raggi subscribes to it, I totally don't.
Zak. Zak does the D&D with Porn Stars blog. I don't read it that often, not because it isn't interesting but because it's not on the top list of things I read. However, what I have noticed about Zak is that the approach to thinking up new ideas often REALLY comes out of left field. His method for handling the open-endedness of a city adventure is really fascinating. Unlike Jim Raggi's idea that gaming is art, Zak takes this from another direction, which is that an artistic APPROACH can spur creativity for running a game that's completely beer and pretzels fun. That's different.
Stuart Marshall. The statement that D&D is a challenge to player skill rather than to the character sheet is brilliant, and I think it originates with Stuart. Many people, even since the 70s, have articulated thoughts around this concept, but I think Stuart gets credit for a nutshell statement ... hitting the nail on the head, if you'll pardon the dual metaphor.
Tim Kask. The original proponent of the idea that fewer and vaguer rules create a qualitative as well as a quantitative difference with a game that contains rules for more situations. I picked up on this in my Quick Primer, but Tim was saying in the forewords of the original D&D booklets.
EDIT: I forgot to mention why Tim is pushing a boundary NOW with something he wrote in the 1970s. It's because everyone ignored him back then, and the concept was swept away for decades. In the frenetic excitement of the days when D&D was as popular as the pet rock, the dippy bird, the "Keep on Truckin'" guy and the "You want it when?" picture, not to mention the Loni Anderson poster, people wanted more ideas, and those took the form of rules, because it's a game. Games are developed with rules, right? I can picture the tension that Tim's viewpoint would have been on this, and when he says that he helped EGG assemble the materials for AD&D ... I tend to think that Tim wasn't seen as a team player on that project, and clearly some tension arose between him and TSR right at that point in time. I have no idea what happened, but if Tim's ideas were as well formulated then as they are now, I can't imagine that he was seen as a cheerful proponent of the codification of OD&D into AD&D. The guys who could say for sure probably wouldn't choose to say it, and it's just my completely unfounded theory.
Okay, that's the thought of the day. I totally forgot a bunch of people, I'm sure. Specifically, though, I left out James M's categorization of D&D history into golden ages and such because it's largely historical rather than prescriptive. Just because I'm sure lots of people will call me on that. Also left out Jeff Rients, Michael Curtis, etc. That isn't a put-down, it's that their writing doesn't fit the specific category of game-thinking I'm talking about here.
But I'm sure I actually did leave some people out by accident. Point them out.
EDIT EDIT EDIT
And it has been pointed out to me that I forgot Gabor Lux, aka Melan. Melan is perhaps the strongest advocate of two areas - the "weird fantasy" feel of Weird Tales and the baroque, pulp CAS/Lovecraft type of writing as it intersects with gaming being the first. The second is mapmaking - his advocacy of the gridless map and the non-linear pathway in the physical structure of an adventure. What an omission. Sorry, Gabor.