This moves RPGs out of the bounds of "games" as a field for many players. Playing a game I don't know the rules to (unless we're talking about the sort of puzzle game where learning the rules is in fact the gameplay loop) is something I've described before as a special kind of hell. This takes us into the territory not of people playing a game together, but one person using a game as a mechanism to do something else; an activity that clearly can and does have value, but isn't the same, and doesn't engage the same faculties I'd use to to play any other kind of game.
I've argued before this is overrated as the defining feature of RPGs, and I don't think it's necessary to constitute one. I think the far more important feature of the RPG is that their unbounded in resolution and time; the mechanisms might spell out a few loss conditions, but when the game ends and what the goal is are negotiated by the players and not fixed by the game systems; moreover, they continue to be negotiated and changed over the course of play.
I can see the appeal of "I can declare any action" (though of course, you can't really, you can declare any action within the bounds of what the other players, including the GM, can conceive and approve of), but I don't think it's essential.