If a player doesn't like it, he's welcome to politely leave the game. And obviously, if a whole group doesn't like it, the DM can either change or can lose the group.
I gave an example of this upthread. The only response I have had is [MENTION=69074]Cyberen[/MENTION] implying I am an unreasonable player. [MENTION=67338]GMforPowergamers[/MENTION] has noted, upthread, similar responses to his previous anecdotes of leaving games. And [MENTION=22779]Hussar[/MENTION] has also been criticised, in past threads discussing similar issues, for indicating a willingness to walk from a game that he didn't care for.
Assuming that the GM does not want to lose the player/group, how is s/he expected to know that change is needed? To me, it seems the best way is for the players to let him/her know, either by direct communication or by implicit signals about what they are enjoying and what they are not enjoying.
In my home-brew campaign setting, I reworked the MM goblin.
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Four of my five players were old hands at D&D. Two of them had been playing (on and off) since 1e. I did not tell the players in advance that I'd used the various monster-building guidelines and rules in the MM and DMG to create my goblins. I just sprung them on the party (when they were first level, at that). So, is this messing with 'D&D canon' (which, if indeed there were such a thing, we know just from this thread, would be subject to change with every edition)? If so, should I have forewarned the players of this deviation from official monster listings? Should I explain to players before introducing a new creature to the game how that creature works? If I don't, am I not diverging from 'canon'?
What does a first level PC know about 'canon', by the way?
I don't look at the issue from the point of view of "1st level PCs" - as I see it, the issue is one about player expectations, it's not an issue about the PCs in-game knowledge.
With that autobiographical detail out of the way, I can't answer your (rhetorical/hypothetical) question about what you should have done - I don't know you (other than some interesting posts on these boards!), I don't know your players, I don't know your game.
But I can give an anecdote of my own. When I started my current 4e game, I gave the players a few instructions:
(1) We'll be using default 4e lore from the PHB, MM and DMG;
(2) In your PC's backstory, please give your PC one person/thing to which s/he is loyal;
(3) In your PC's backtory, please give your PC a reason to be ready to fight goblins.
I set out to make goblins a focus of the game, at least at low-levels, within an express framing of default lore. I regarded myself, then, as obliged to present goblins as prospective enemies which dovetailed into the PCs' backstories. When the players wrote backstories involving previous wars between goblins and dwarves, or wrote backstories involving goblins sacking towns and villages, I had to incorporate all that too.
But no-one wrote a backstory that connected to goblins having a +2 bonus to Thievery by the default rules, and goblin thievery has never come up in the campaign, even after 6 years of playing it. That's a detail that I don't regard myself as bound by (though nor have I deviated from it - it's basically irrelevant to how the game has unfolded).
If one of the players had built a thief PC whose guild was a rival to the notorious house-breaking second-story goblins, this issue might have been different.
Having good judgement on these sorts of things is part of GMing skill, I think. Like most matters of judgement, I think it is very hard to give hard-and-fast rules about what should or shouldn't be done. An ability to grasp the situation, what is at stake in it for oneself and others, what is possible consistent with those stakes, and bringing others along with a decisions, are all part of it. (I'm reminding myself of Selznick's
Leadership in Administration.)
One of the primary responsibilities, and primary joys, of DMing is world-building and setting design.
For you, perhaps. For me these are secondary - for me the primary joy is in adventure/scenario design, and adjudicating the game in play. (This hasn't always been true. Like many people, my tastes have changd over the years. I can't predict how they might change further into the future.) I think [MENTION=22779]Hussar[/MENTION] has said in the past that he has very little interest in world-building as a GM activity.
For instance, when I told the players that their PCs must have one loyalty, I was leaving it open to them to build (or foreground) those parts of the setting: what I got back was two PCs loyal to the Raven Queen, one loyal to the ideals of the fey and the Elven gods, one loyal to the dwarven strongholds and his mother, and one loyal to the memory of his fallen city. The joy for me was finding ways to make these loyalties a focus of play, to develop connections between them, to push the players to make decisions around them, to see what sorts of conflicts might exist between them.
the DM has to know his players, and the players have to know the DM. But that goes back to what I said about "minimum trust." Unless it's a brand new group that I'm just starting to get to know, I always take into account what I believe my players will enjoy when designing a new campaign. AFAIAC, that's such a basic part of the process that it doesn't even warrant being called out. It kinda goes without saying, I think, that a DM who doesn't consider his friends' preferences when designing a campaign isn't going to have players very long.
You are framing this in terms of campaign design. Why is scenario design, or encounter design, in a radically different category?
When I conceive of scenarios, or encounters, I try to design things that the players will enjoy. I've never had a player who cared about the habitat of manticores, but if I did, I would have regard to that. (And how would I know unless the player told me? - perhaps, in the limit case, by voicing an opinion about an encounter I was in the process of framing.) The nearest example I can think of to the manticore one is when, in my 4e game, I was starting to frame an encounter in which the PCs had to overcome some sort of opposition/objection from some NPCs in relation to XYZ. (I can't remember the details at the moment.) One of the players reminded me that, last session, the players had succeeded in a skill challenge in relation to these NPCs, which meant that the issue of XYZ was already sorted between them. I agreed, and corrected my framing to reflect that. I regard that sort of correction, when a player points to a problem with an encounter framing, as part and parcel of good GMing.