A key observation is that the DMG is only meant to be known by one player, and are also very clearly stated to be guidlines and suggestions rather than rules. It hence serves no role in setting expectations as to what the activity would entail when inviting players to "play D&D".
I don't think I agree with that. I mean, I get why you say it, but how many people try their hand at GMing? I know it's not all players, but certainly enough that such an expectation is unrealistic.
More importantly, the DMG is the book for the DM, who is going to be adjudicating interaction with NPCs. It's the source of guidance for DMs. The process is clearly described... but no support for it is given in any way.
Let me put it this way. If each NPC in an adventure, if each monster entry in one of the monster books, were given Traits, Ideals, Bonds, and Flaws, along with maybe the occasional suggestion about how such can be leveraged by players.... do you think that the rule would see more use than it currently sees? If the DMG offered this as a rule without preceding it with a wormy paragraph like "some people like this, some like that, some people like to do a little of both" how much more attention would be paid to it?
Imagine if combat rules were equally mushy. Would that help the game?
However the players handbook is the one book assumed to be available to everyone, and hence what is written in it is the basis of what can be reasonably expected when a group of people agree to "play D&D". This hence provide the baseline expectations. Deviations from what is written in that book should in general be clarified up front, or else you can be accused of deception.
Again, I disagree. Not because I think what you've said is wrong, but instead because based on what's in the PHB, there is no baseline for some key aspects of play.
Obviously, it gives a good idea of a lot that can be expected, absolutely, but I also think there are significant blank spots which if not addressed can lead to mismatched expectations by participants.
Character creation, combat rules and content with defined interactions with these two subsystems make up the wast majority of every PHB. This is why I consider them key situations.
Agreed.
How would you feel about them if they were less clear? I mean, I know many folks who hate milestone leveling because it basically leaves it entirely up to the GM when you level, and I can totally understand that criticism.
However there also typically are some other minor things like overland travel speeds and the light and duration characteristics of a torch. As at least minor changes to these are unlikely to have drastic effect on play, and their not so prominent place in the PHB, I think most groups will be fine with rule zeroing on these - as long as the changes is not obviously drastically changing the nature of play as advertised.
And I think the notion of advertisement is critical here. If someone join a game without knowing more about it than "we are a group playing D&D, come join!" You really do not know much about what they are doing. Hence follow up info like "we play this adventure" or "we do a home brew setting" or " we play in a colaborately created setting" is commonly expected to narrow the expectation. Indeed we have invented special vocabulary to communicating further narrowing of the activity like "pillars of play" balance, degree of character focus or "hexcrawl" vs "pontcrawl" vs "dungeon crawl". And while each of these qualifiers help with setting proper expectations for how the game will be, it is hard to compete with an entire book where an author has put their mind to accuratly trying to describe a style of play trough mechanics..
Sure being upfront about all this stuff is an important element. I just think it's hard to address them all because so many are not defined in the rules, and so many people take certain ways as a given that they don't address it.
Now, that's not to say that will always be an issue. Just that it's work the DM or group will have to do what the books could have.