Can I hold you guys to this the next time a "Say Yes" discussion comes up, and it is claimed that it doesn't mean a DM can't say No to knocking a snake prone, playing a Warforged Ninja in a PotC setting, etc.?
Neither "Say Yes" nor "Say No" should be the default.
The default should be: "Say what you think will make a better game".
Well, I think you may be mistaking the difference between a default and an asbolute. The concept behind "Say Yes" is a reminder to the DM to carefully consider whether there is a good reason to prevent a PC from attemping something interesting or playing an unexpected role.
That said... yeah, I'm mostly in agreement. Reasons to approve or disapprove of a choice will change from one DM to the next, one game to the next, there shouldn't be any set absolute one must adhere to.
And I don't think that is what the "Say Yes" advice is intended to be - it is simply a reminder to think twice before making such decisions, and make sure that whatever your reasoning (preserving integrity of the setting, internal consistency of the combat, avoiding setting precedents for unbalanced stunts in the future, etc) does intend weigh in stronger than approving something that would provide a more entertaining experience for the player.
Now, ever since 3.0, I've been hearing how WotC-D&D is more transparent than TSR-D&D, thus making things easier to judge. It seems that each new version is more transparent than the last.
One would think that, if this were true, it would be easier to tinker with WotC-D&D than TSR-D&D.
But, over and over again, WotC-D&D advocates say that tinkering with even such a small thing as whether or not a snake can be knocked prone is going to somehow throw the game off kilter.
Well, for myself, I think such claims are silly. But I also don't see them crop up all that often. Is it possible you are misreading some similar concepts as that sort of argument?
Being able to knock snakes prone, by default, isn't really an issue of balance within 4E, but one of philosophy. 'Avoid removing PC capabilities without good reason.'
It isn't that the game breaks down if the polearm wielding fighter can no longer trip snakes. It is that one player might feel sidelined if he's built to flip enemies over and take advantage of their weakness, and suddenly they have entire adventures in which everything is immune to his core function.
The other claim that I often see, which might tie into this, is that while it won't disrupt the balance, it does set the stage for a lot of nit-picking. You rule that snakes can't be knocked prone, and one PC instead now wants to argue that they should instead always count as being prone, just that they don't suffer the normal penalties for being prone. This means that his feat, Headsman's Chop - which gives damage bonuses when using an axe or heavy blade to attack someone while prone - would still apply, even if the snake didn't grant combat advantage or have difficulties with moving.
His argument may even make sense - one imagines someone with such a skill would be perfectly able to put it to good use against your standard snake slithering along the ground - but then you open up the situation to even more discussion. Can the snake turn off this 'counts as prone' shtick if it coils up? Does that reduce its reach when it does so?
You end up getting into a lot of minutia. I don't think any of it - pretty much any decision made at any point in this process - is likely to unbalance the game. But it will slow it down, and once you set the precedent of letting what makes sense override what is written in the rules, you risk everyone having a different opinion for how 'what makes sense' should actually work in game terms, and Knights of the Dinner Style arguing following from there.
Now, I'm not saying every rules call will lead to such antics, nor that one should avoid making a call just for fear of player disagreement. But I think it is one of the reasons offered for trying to adhere to a strict rules system in order to ensure shared expectations by everyone at the table. And that this sort of point is put forward more often than claims that the slightest adjustment to some critter's abilities in 4E will somehow result in catastrophic damage to the balance of the game system.