Were I to focus on one of the less obvious ways that older versions of Dungeons & Dragons differ its latter day descendants is in the matter of henchmen. Henchmen were an important part of old school play, as evidenced by the very existence of the Charisma score. Consequently, the Dungeon Masters Guide contains many sections devoted to this topic, such as page 34, where Gygax says the following:
Henchmen, whether male or female, are greatly desired by the discerning players, for they usually spell the difference between failure and success in the long term view.
If anyone ever doubts the importance of henchmen, let him contend with this statement. What I find most interesting here are two things. First, Gygax says that henchmen are "greatly desired by the discerning players," not characters. It's possible that is simply a casual conflation of player for player character that can sometimes be found in early RPG writing – it happens a lot in Empire of the Petal Throne, for example – I'm not wholly convinced of that. Gygax was very interested in "skillful" play and regularly stated that D&D (and RPGs more generally) was something at which one could become better with time and practice. My guess is that's what's going on here when he references players. Second, his use of "long term view" is a reminder that Gygax, like many early roleplayers, was strongly focused on the campaign as the ultimate expression of the game.
Gygax goes on:
They are useful in individual adventures as a safety measure against the machinations of rival player characters, provide strength to the character and his or her stronghold, and lastly serve as a means of adventuring when the player character is unable.
The first point he raises here is probably outside the experience of many people. The idea that players might, through their characters, scheme against one another is unusual today, at least in Dungeons & Dragons. I can assure you that it was not in the early days of the hobby; even I, who didn't start playing until late 1979, saw plenty of evidence of this, not only in my own friend group but also among other groups I encountered. It certainly wasn't endemic behavior by any means, but it certainly happened enough that Gygax draws attention to the value of henchmen as a guard against it.
Strongholds, too, were more common in the early days of gaming. They are an essential part of D&D's lost endgame, something that was still emphasized as late as Mentzer's BECMI boxed sets, so it's little wonder that Gygax makes note of them here. Equally interesting to me is the notion that one could play a henchmen as a kind of "substitute player character" in circumstances when one's actual character is unavailable. This practice was widespread in my youth and is a way one can recognize a rich and well-developed campaign. My House of Worms Empire of the Petal Throne campaign is full of instances where the players have taken up the roles of NPCs associated with their PCs when it was, for various reasons, impossible to play the PCs.
Because they are so useful, and because they are typically so devoted, there are charisma limitations as to how many henchmen a PC is able to attract.
It's fascinating, from the vantage point of more than four decades in the future, that Gygax simply takes it as given that henchmen are "so useful." I don't disagree with that perspective in the slightest, only that it's not one that's commonplace nowadays. Does contemporary D&D even mention henchmen – or would that be henchfolk? – or describe Charisma in a way that relates to the attracting of followers? I honestly don't know, but I'd be delighted to learn that it does, since, as this section of the DMG makes clear, Gygax at least considered them a significant aspect of play for "discerning players."