I'd like to mention though, that when I finished through Braid I ended up with the feeling that it's primary goal was not to make me look at pretty images or listen to some music or even to make me read the puny bits of story. I don't think it's purpose was for me to finish it either.
The gameplay was original and highly enjoyable, but that might not have been the point still. It sure pulls you through the game, but the graphics and the music and the story contribute to that aspect in their own right.
I ended up thinking that the game's purpose was to communicate a feeling, a certain moment frozen in time when you realize for the first time that you've made a mistake, that what you have come to believe about the world is false or backward, and you're, in essence, a devastating force. This is the moment when you want to take it all back, to have made different choices.
And I think I'm not alone in feeling that. I also happen to think that these moments of direct communication are what art is about, and that they might be achieved with simpler means – a song, perhaps, or a poem.
Ebert hasn't had any exposure to this kind of experiences, it seems to me.