D&D changed a lot both in tone and in scope.
The tone in general is lighter and lighter going into the ironic or highly fantastic (and not necessarily fantasy). We went from light rule and DM's centered arbitration (OD&D to 2ed) to players' Agencies (with capital A) in 3ed and 4ed. To a more balanced approach in 5ed.
Still, the major change are the players and the assumptions about the game itself.
Early on, the basic unit of play was the group. No matter the players or the characters the adventuring party was the most important thing. It's inherent components were important, for sure, but never as much as the group itself. Character death was expected and the heavy use of henchmen and hirelings were there to make the player jump in the game ASAP with an already established character that was promoted from henchmen to "main protagonist" or simply to a played character. With a high death ratio, the stories were oriented toward the group and not the individuals. There were so many articles about how to build a successful group or simply how to be useful to your party that it was almost ridiculous.
This started to change around mid to late 1ed. The focus started to be put on the character itself and not the group. The group was still important, but more and more articles and even official books started to build on character development. Books such as Wilderness survival guide and Dungeoneer Survival guide were expanding the character sheet to better round up both the rules and the characters. It slowed down with 2ed where the group was for a little while, back on the main spot light. But soon after, we started to see more and more player options and less and less adventures being build. Adventures were now usually to sell the new campaign boxes and the most successful campaign worlds would get the "world book" treatment where more and more cultures were detailed to help creating different characters from the norm (and here mechanics were always the same, it was more background oriented than anything else). And of course, the various complete "insert class, race, culture or whatever" started to appear right there... The group was detroned as the most important thing in favour of the character. Around this time, monsters as playable character with character classes starts to appear (before that, it was simply homebrew and nothing was ever official).
In 3rd edition, the shift was complete. Never was character builds so detailed with rules and shenanigans to make a character feel special. To ensure that a character was to stay special, HP started to get higher and higher thus ensuring the survival of the character so that the player would feel "special" and "unique" and could develop fully during leveling. Unfortunately, balance was thrown to the four winds and min/maxer soon found out the bane of all games. The 5 minute work day (or 5MWD) and the CODZILLA! Now martial were simply there to ensure that the casters would stay alive until they could become the monsters of players characters that the CODZILLA were... You still needed the martial characters in the early level but as soon as the caster were of sufficient level to become CODZILLAs, the 5MWD would get on and martial character were at best 2nd class character there to watch over the rest period of the casters. Character playable races explodes at mid life of third edition with various books and 3pp...
4th edition tried to "balance" the classes by making them all alike mechanically. Unfortunately, many players were now accustom to being "special" with races, cultures and skills that uniformity was not desired. The famous moto: "Bring the player, not the class" was a direct reference to the inspiration that this idea came from: "World of Warcraft".
Note: "I did love 4ed. No edition warring here. But admitting where some of the inspiration came from in no way diminishes my love for the 4ed. I am simply realistic about what it was and where it was coming from and going to."
This removal of the capacity for each character to be special was ultimately, the downfall of 4ed. Character build had optimal capacity to choose and at each levels, one ability was clearly superior to the others. This led many in the community to flock toward Pathfinder to get not only the philosophy of 3ed but to a system that brought even more character differentiation. The explosion of playable race was big in PF even more so than in 3ed.
5ed brings back the DM as a final arbiter and a refocus to the group as the basic unit to build upon. The rules have been expunged from a lot of fluff but also from a lot of direly need rules. Some of the over simplifications brought more heated debates than what we see in politics... But the players are now in dire need of specialness. The rules so expunged from fluff now allow some customization but not on the level that 3ed allowed rule wise. The players seeking uniqueness must use their imagination and seek DMs' approval to be allowed to make something special. Subclasses, TIB and Background are now the main ways to make a character special as skills and feats have been reduced to a bare minimum (and in the later case are simply optional).
Flash forward of few years and this leads to today where the number of subclasses, races and cultures are booming (again) to give players the "uniqueness" they so desperately seek. The "Mos Eisley Cantina effect" so despised by the old schools way of thinking and old grognards like myself is now omnipresent in the mindset of the younger generations. They want rules that allow them to do whatever they want and whenever they want, regardless of the DM's wishes. The 3ed entitlement of players is far from having been forgotten and it is still haunting the game even to this day. One of my friends, recently took up a new group. Out of 5 character races chosen, none were in the PHB and Background stories were more outlandish than one another. Since he plays in his own campaign world (a world that is almost 50 years in the making), he told them to choose from the PHB (no dragonborn or tiefling either). They categorically refused. He simply took his books, and left. This single event perfectly illustrate that the paradigm changed over the years and that the ghosts of 3ed where the group was simply dropped in favour of player's agencies is still there, stronger than ever (if not more so). And strangely, these players have no trouble being restricted in races, cultures and whatnot in other game systems... It takes quite a DM and players to reach a balance between players' agencies, DM's preparation and group cohesion. Session Zero should be more defined and explained in the PHB. A lot more.
