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, you know what my specific answer is to Can I play a Warforged Ninja in a Fantasy Pirates campaign.

(For those who don't, it's well, hell yes, that would be cool, provided you can come up with a suitably piratey background). Granted, if you said, "play a Warforged Ninja in a Master and Commander setting" I'd say no because it's genre breaking. But, to each his own.
I find that "Say Yes" or at the very least "Yes and.." to be the best way to run a game. This, to me, is the dividing line between Old School and New School games - the diffusion of power from the DM to the entire group. In an Old School game (system independent, I'm talking play style here) the DM is the final word, no matter what.
And, to be 100% fair, this works quite well. You get a single vision of the setting and, presuming the DM isn't a dick, a balanced and fair arbitration of the mechanics. This can work extremely well.
But, New School games tend to reject this paradigm of running the game. Instead of residing all of the responsibility (and blame) for the game in the DM, New School games make all players responsible for the game. Everyone at the table is responsible for making sure that the game is the best game it can be.
Again, this can work very well. If everyone is on board, and the social contract at this table becomes much more important, then the game is going to be the best experience for that table. However, it does place an onus on the players that was missing in the past. In the past, the players were only responsible for themselves. I come to the game, I play my character, and that's all I am responsible for. In New School games, the players are not only responsible for their own characters, but also for the group as a whole.
Obviously, I'm a very New School type DM. I advocate strongly that everyone at the table is responsible for how the game turns out. No final authority exists at this table, beyond what the group as a whole can hash out. Which tends to mean that the rules are taken as the default state unless the whole group wants the rules to change.
If everyone at the table agrees that snakes can't be knocked prone, then, groovy, change the rule. However, if it's only the DM vs the Player, then the default (at my table anyway) is that the rules should stand.
Then again, I play with players I can implicitly trust to make sure that the game will be the best
we can make it. I don't want to play in groups where that is not true.