What is the difference with the free version?

[gordonfreeman01]

This marvelous game has been available for free on PC for years and it's great to see it released on Steam but given that it is paid, what is the difference with the free version? I remember it was released on PS3 a few years ago and that was a paid release but do not know what was enhanced (not that the game needed it though, it's quasi-perfect for what it attempts and succeeds to do). Hopefully this developer's other PS3-exclusive (afaik) music game - a platformer called Sound Shapes - also comes to Steam soon!