While I could sit here and type out a list of all the things good about this game (and there are many), there is one tiny feature that stands out. The "bark button".
The bark button is pretty much what it says, pressing O results in a bark from Ameratsu. I've found myself pressing this button with great gusto, barking at all evil monsters that dare to befoul the land of Nippon. Each and every button press elicits a smile.
How can such a small, throwaway feature, be so instantly rewarding?
The button acts as a singular conduit with which the player can meaningfully communicate their feelings in the world. As a silent protagonist, Ameratsu is spoken to, but is unable to speak back. It's easy to feel as if you are being taken along on a sightseeing tour. The bark button could signify frustration at another character, a challenge to an enemy, or a howl of upset. It means all of these things, and when the player presses it, the feeling that the player's emotions are being communicated to the game world is overwhelming.
Many players already do this: the unnecessary, gratuitous car horns in many a driving game are the same mechanic.
This is not to be confused with the emotive stances taken by games with deeper narratives, such as Mass Effect. While the option is there to be aggressive, docile or assertive; games such as these force these choices to affect the course of gameplay. Okami forces no such repercussions on the player, it is a one-way conduit, a way of showing the game world that you mean business, but without a potentially unintended consequence. This allows the button to be pressed time and again, bolstering the player narrative.
Action games stand to gain most from inspiring emotes. How about a roar of rage, a cheer of victory or a trash can kick of frustration? It's important to allow players to properly "voice" their own feelings in the game world, to foster a connection between player and avatar.
Okami was lucky: there were buttons to spare, and the character was abstracted in such a way that a bark could signify many different feelings. In reality, it's unlikely many games will be able to find that free button, and would need some contextual recognition of what the player was likely feeling. But if developers jump these hurdles, that button might well turn out to be the most direct, gratifying interface element they'll implement.