I'm not digging our my 1e DMG for this, but my guess is that the real difference for modern design isn't that principles exist, but that the statement is made clearly.
Yes, that is what I posted:
It's not a mechanic, but here's a thing that strikes me as modern when it is found in a RPG: a clear statement of GMing principles.
A related thing, which also generally strikes me as modern, is a clear statement of how actions should be resolved, which makes reference to principles as part of the process.
I think one of the reasons that Moldvay Basic has such a strong contemporary reputation is because there is a reasonable clear statement of GMing principles, in Chapter 8. AD&D doesn't have the same clarity, and I think there is some tension in the DMG. Contrast, for instance, invocations against "Monty Haul" play with this, on p 90: "The 'reality' AD&D seeks to create through roleplaying is that of the mythical heroes such as Conan, Fafhrd and the Gray Mouser, Kothar, Elric, and their ilk. When treasure is spoken of, it is more stirring when participants know it to be TREASURE!"
Clearly stating principles can also help establish clear principles.
What I do think is fairly modern is a clear set of statements of how to best be a player and a GM such that the game is successful in realizing what the designers intended it to do, and also telling you what it intends to do during play.
In my post I mentioned GMing principles in particular. Player principles I think are found more often even in "non-modern" RPGs. For instance, Gygax's AD&D has a clear statement of player principles in (i) the PHB and DMG discussion of class functions (which
@Fenris-77 and I were discussing upthread), and (ii) the PHB section on Successful Adventures.
I actually think the PHB is better on this than Moldvay Basic. When the two are combined, the upshot is a pretty good account of the roles of the GM and the players. If they were reproduced on a couple of pages each, with clear dot points and/or bolding, I think that would feel quite modern (at least to me).
