Braid's purpose has nothing to do with the twist at the end, that's just a cool conclusion. The story, visuals, and music is not what makes Braid a good work of art. It's that it takes full advantage of the medium games - that is, interactivity - and uses in a way that imparts a genuine learning and growing experience for the player, and in a way that's thematically consistent wit the game. Think about that - the theme of the game's mechanics (time manipulation) works consistently with it's aesthetic theme (memories, regret). That's an excellent first step to helping transform the medium of games into a bit more of an artistic one.