Interesting reads.
I think there's two points I where I see the biggest changes.
D&D as a genre has moved through time similar to how music genres work. If you take 80's hair metal as a genre - the Poison, Def Leppard, sort of thing - you have a pretty small, very popular, but narrow music genre. But, it got really popular. So, it expanded. You then get groups like Guns and Roses adding in new sounds. Then it grows into Alternative with Nirvana. Now, it's such a huge genre that it's subdivided into a bajillion little niche genres, each with it's own fans and lots of crossover. Once upon a time, adding rap and RunDMC to Aerosmith was innovative and new. Now, you have groups like Linkin' Park where rap/rock/alternative is the norm.
D&D evolved in kind of the same way. Way back in the early days, say, pre-1981(ish), most games were pretty similar - the game was new and we simply hadn't explored out that far. Then you have the fad days and the burn out of the 80's where you see all sorts of experimentation - the birth of the adventure path/scripted campaign with Dragonlance, for example.
Forward into the 90's and you see all sorts of non-D&D games all over the place. And, IMO, these games were getting a fair bit of play. FASA (of Battletech fame) was a pretty damn large RPG company. White Wolf was also. You had all these mid-sized (for RPG companies) companies pumping out all sorts of stuff that was then circling back into D&D. The notion of stronger emphasis on (sorry, not sure what the right word for this is) more theatrical style play. More improv acting style focusing on character and personality building. Which in turn meant making the game a lot less instantly lethal since that would get in the way of character development.
Then you get into 2000's and the explosion of the game and the Internet. Both have HUGE impacts on the game. For one, you have all this 3rd party stuff coming out for D&D, which leads to all sorts of discussion about the game that was virtually impossible before. The notion of the "professional game developer" rises. Massive increases in production values. And, because of all of this, a huge fracturing of fans into niche's. Some niches, like the OSR for example, are quite large, but, everyone has their own little niche and approaches to the game.
4e then makes the mistake of strongly catering to a specific niche - the public gamer - the gamer that plays with strangers. And it brings in all sorts of imagery and concepts tied to that. The characters are Big Damn Heroes because, when you play with strangers for 2 or 3 hours on a Wednesday evening, or for 1 hour at lunch break, you don't have all this time to develop characters the way you would when playing for 9 hours on a Saturday night with friends. So, WotC had to try to step in and not only create a system for play but a system where strangers could play what they wanted to play without everything having to be vetted by the DM all the time.
Then 5e comes along and rolls some of that back. They soften the Big Damn Hero thing somewhat - but, a lot of the aesthetics are still there. The Tolkienesque races are fading and the newer ones are getting more and more popular. Magic is far closer to Harry Potter than earlier fantasy - it's entirely plausible that your group includes all casters (of varying levels of casting ability) and quite probably that every single round of every single combat encounter (and probably most non-combat encounters as well) will feature spells and quite probably multiple spells.
To me, the shift from Tolkien to Harry Potter style fantasy is probably the biggest change yet. Back in the early days, you might have a single spell cast in an entire encounter and many encounters wouldn't have any. Now, the first thing players look at is for what spell they are going to cast this time. Watch your next session. Count how many spells are cast in a single session. It's astonishing. The dominance of the spell system in D&D is, to me, the single biggest change in the game from early days to now.
